I'll try to keep this concise. First of all, having a professional affiliation (even as a graduate student) offers a certain amount of cred when attempting to access primary documents. Most of the archives I had traveled to demanded a letter from my advisor/institution before they allowed me to put my grubby paws on their precious 300-400 year old manuscripts. In addition, there are expensive/rare secondary materials that often cannot be easily/cheaply obtained by a non-academic historian. Even when affiliated with a fourth tier institution, materials can still be ordered via interlibrary loan, all because that institution is willing to vouch for its students or faculty. The instruction offered in a collegial academic setting is perhaps the difference between a professionally trained and an amateur historian. In my experience, amateur historians are rarely capable of utilizing theory, historicizing, interrogating the source materials or taking an interdisciplinary approach. Furthermore, many cannot distinguish between peer-reviewed, credible scholarly work and an ahistorical hatchet-job that some conspiracy nut may have written with a crayon on a roll of toilet paper, citing YouTube videos as source material. In graduate school, such a diverse skill set can be acquired in a relatively efficient manner - and even then, it's still a baptism by fire with a sizable attrition rate. To undertake such a task on one's own with no guidance from a whole body professionals (not just historians, but academics across varied disciplines and specializations) seems beyond herculean. Finally, there's the benefit of networking. When I was just entering graduate school, my advisor informed me that in order to be an effective scholar in my chosen field, I would not only have to acquire a series of dead languages, I would then have to learn the appropriate paleographical skills in order unlock period manuscripts, which were located in these and these particular archives. Fortunately, my institution offered a course the said dead languages, which I could then adapt to my research via a particular summer program, the director of which was a onetime colleague of my advisor. The said director was then able to introduce me to other scholars who worked in a field that was closely related to my interest. One of these invited me to a conference, where after a successful presentation (according to my academic peers), I was invited to contribute an article to a journal she just happened to be the chief editor of. Once submitted, this article will be reviewed by other scholarly experts in my field before being published. This ability (and means) to produce credible scholarship is perhaps the ultimate difference between a professional historian and an amateur historian.