what was the point of slave names? was it really just so overseers didn’t have to try and remember foreign names?

There are a few factors to consider in answering this question.

On the one hand, you are probably right that there is a measure of practicality involved in the practice of giving African slaves more familiar names; from a logistical perspective, it's simply easier and more straight-forward if a planter or overseer doesn't have to bother getting to grips with many exotic names. Certainly, names were chosen by planters and overseers in such a fashion that reflects a desire to distinguish between individual labourers as simply and easily as possible.

We must also be mindful that western English naming conventions - (forename -> middle name(s) -> surname) are far from universal. Even within the wider western world naming conventions are inconsistent; in Spanish, for instance, it is traditional for a child to have two surnames (one from each parent), whilst many Roman Catholics take a confirmation name after their First Communion. In many West African cultures, it is not uncommon for an individual to have many names claimed at various stages in life, some of which may replace older names. Personal names given shortly after birth could relate all manner of cultural concepts or familial experiences; some names might be reserved for certain formal contexts and others for more relaxed occasions, much like we might use a surname at a job interview but a nickname with friends. So we should perhaps be cautious in assuming that the practice of renaming slaves represents a simple replacement of one personal name with a new one - it is questionable to what extent an individual from West Africa could maintain their full identity even if this had not been a feature of racial slavery.

Really though, the practice of imposing new names on the enslaved is about is best understood through the frameworks of power and racial stratification.

Though the significance of particular kinds of names might vary from culture to culture, they nonetheless have significance to us as individuals. In the western world, if someone adopts a new name, we tend to think of them as adopting a fake identity - hiding their 'real' name - rather than merely changing it. In other cultures where names are more malleable, their changing nonetheless reflects significant developments in the life of the individual, family or community. Our names are part of our sense of identity, our sense of self. They usually connect us in some way to our family, being given by our parents and possibly being named in honour of a valued relative, and may have some kind of cultural significance. Though it may not seem like it to many in the western world today, our names do have a certain psychological and social significance that by far transcends the purpose of making it easy to identify individuals.

To impose a new name on a slave then is fundamentally an exercise of power in extreme. Orlando Patterson famously described the condition of slavery as a kind of 'social death', with enslavement a process that is designed to alienate the victim from all bonds of kinship, family and community. Changing a slave's name - replacing their old identity, rich with significance, with a new one - is one of the ways in which that alienation can be achieved. By giving a slave a new name, the slave's owner was trying to subvert and destroy their sense of self-worth and identity, and replace those things with a new social identity that oriented the slave's life around the master. It was an expression of power and domination on the part of the owner, a means by which their total authority - and right to exercise that authority - was demonstrated. It is an act of degradation, humiliation and social violence. The acceptance of such a new identity on the part of the enslaved is by implication symbolic of their supposed acceptance - at least in the eyes of the owner - of their new status. It was all part of the mechanism by which total domination could be achieved, and one of the many ways in which slave owners sought to destroy African identity.

In racial terms as well, with the emergence the racial paradigm that justified slavery, it certainly makes sense that slave owners would have had little to no interest in preserving traces of an African culture they found backwards and uncivilised. The refusal to accept the legitimacy of African names, whilst primarily a means of forcefully demolishing African identity, could also be taken as reflective of the contempt for African people that developed in white racial discourse. Certainly, planters very much wanted their slaves to have some measure of racial identity in the sense that they wanted them to conflate black with slave; but they certainly did not want them to develop any kind of sense of broader African identity per se. That was seen as a road to rebellion and ruin, and thus the pro-active subversion of retained elements of West African culture was a means of both ensuring 'loyalty' and enforcing the emerging racial hierarchy in North American societies.

Other considerations may have come into play also - a Christianising agenda, for instance, may have motivated some to impose more Christian or Biblical names. It should be noted though that slave owners with an interest in large-scale Christianisation were few and far between until the late antebellum period (about 1830 onwards); the use of Christian names before this is probably much more cultural than religious, and though certainly some early slaveholders were believers in Christianisation, they were probably the exception, not the rule. In the early period in particular, we see many slaves who have names taken from contemporary works of culture - from classical literature or mythology, or from popular plays or books in circulation at the time. The selection of some of these names may have been tongue-in-cheek or intended to be derogatory.

So the significance goes beyond simply distinguishing between slaves or making logistics easier, though that's certainly a factor - the practice of renaming slaves is an expression of the power the slave-holder has over the enslaved, and reflects a desire to destroy as much as possible any sense of African or individual identity, replacing them with new identities in which the slave's sense of self and integrity is inseparable from their status as the property of another Human being.

There are two big caveats to add to this, though. The first is that just because that was the goal does not mean that they were by any means wholly successful. Attempts to subvert African culture and tradition had very mixed success and naming traditions are no different. Whilst in smaller slave-holding operations it may have been less plausible, certainly those who found themselves on plantation communities had plenty of opportunity to use their own name in their own lives and their slave name around white people. We certainly know that many slaves resisted attempts to rename them throughout their lives. Likewise, we do have some evidence of slaves adapting western naming conventions to their own naming traditions, for instance by adopting secret surnames or family names that forged new bonds of kinship between families and communities. See this answer here for more.

The second caveat is that it is honestly unclear to what extent and how consistently slave owners continued to exercise authority over the naming of slaves beyond those imported from Africa. The nature of slavery means that it is not always easy for us to determine how most slaves found their name. When a new slave's birth is recorded in a plantation ledger, is this the name the owner has been given by the parents, or is it a name they have opted to impose on the child? If the latter, that does not preclude the possibility that parents will have still nonetheless given their children a name of their own devising which they may have identified more with. Certainly, we do know some planters absolutely imposed names on slaves born on their land; we have records even of overseers, planters or their spouses visiting slave women shortly after birth and naming their children there and then.

But we also have evidence of slave-naming patterns that defy what we would expect if they were always imposed by whites: names that signify kinship across generations, often with clear African heritage, and names that repeat within the same generation. It does not make much sense to give two slaves the same name if you are a slave-owner who wants to distinguish between them; it makes even less sense for you to encourage bonds of kinship through familial naming. Certainly in both the United States and the British Caribbean, the norm seems to have been for slave families to name their own children with names that had some cultural or familial significance to them; what is not clear is to what extent these names corresponded to or overlapped with the names recorded in plantation records. Historians have made cases both for general harmony and for complete disconnect between the two. The truth probably lies somewhere between the two, though the pattern seems to favour black self-determination playing a greater role in naming.

Where owners did retain naming rights and sought to impose them, their motivations were essentially the same. In the United States, some may have also been motivated by the pursuit of realising a paternalistic ideology, whereby slaves were construed as being children in a large, perverse kind of family in which white planter-owners were the metaphorical 'father' of all on the plantation - and by extension named their slave 'children' as they would their own. Ultimately though, exercising control over the naming of slaves would be just another way in which plantation owners sought to enforce their power and authority, whilst degrading the autonomy and Humanity of those they enslaved.