The Laws of Manu have been used as justification for a number of laws and policies that many would consider detrimental to human rights in modern times. During the time of their writing, they were not really ever implemented fully by any king, especially the non-Hindu ones. Indeed, the Buddhist kings seem to have respected it to an extent, but preferred to follow their own traditions of statecraft and war as prescribed in the various Sūtras of their traditions.

The Laws of Manu were likely written over a long period of time by conservative Brahmins and thus represent an idealized society in the terms of traditional Brahminism with an emphasis on ritual, purity, and the importance of social barriers. While some parts are quite progressive for their time, others advocate a more conservative view of Varna or women's rights. This is likely the product of it as an evolving document.

When we look at the Laws of Manu as they were initially written, it is clear that it wasn't a lawbook, per se, but a religious and ethical document that changed with the time it was in. It was never regarded as a Hindu lawbook until the British and really only gained real popularity under the Mughals. Similarly, additions creep in long after it's initial codifications and passages related to certain ideas and deities work their way in until the Mughal era, and perhaps beyond.

This isn't to say that it had no importance among Hindus in India. That would be wrong to say. Rather, certain concepts appear to have been quite influential on philosophical thought and statecraft. It's advocacy for Ahiṃsā represented a shift in importance for the Brahminical religion from animal sacrifice to notions of purity through non-violence. Similarly, it affected the ideas of rulership and war, especially among Hindu rulers by borrowing from other developments in Indian thought and also the Bhagavad Gita. Not only this, but the cosmology of the Manusmṛti and myths contained within were popular among the common people to an extent, though I can't imagine the Shudras were happy about their place as advocated by it.

Given this, it paints an interesting picture about the importance of the Laws on Manu on Indian society, with it occupying a somewhat nebulous place in ancient Indian history. On one hand, it was never implimented as a law book and probably was not read outside of the elite and the Brahmins, on the other, the cosmology was widespread and ethical practices advocated by it were certainly used by the common people. Thus, we can see it as something akin to other ethical treatises in history, rather than a concrete set of laws.

It was the British who first used the Laws of Manu as actual, well, laws. This was in part based on a misunderstanding on their part, thinking that the Laws of Manu were akin to Sharia for Muslims (which they used to govern the Muslim population in India). Unfortunately, the British also notice contradictions in the text and almost always went for the more conservative interpretation of the text, especially with regards to Varna, worsening the caste problem in India and taking a more hardline stance than previous rulers ever had with regards to caste administration on an empire level. That said, their use actually improved caste for some in princely states, which under the Mughals and later the Maratha Empire, but on the whole, it was the rigid enforcement of caste politics based on the Jati system and the Manusmṛti that codified caste as we think of it today.

In an ironic twist, the constitution of India is sometimes called, "The second Laws of Manu" and it's writer, Bhimrao Ambedkar, is sometimes called "the second Manu" (ignoring the fact that "Manu" comes from the Sanskrit word for "human"). The irony of this comes from the fact that the Indian constitution pretty much ignores or actively goes against the Manusmṛti most of the time and Bhimrao Ambedkar was not only not a Brahmin, but a Dalit. However, there are ideas in the Laws of Manu which did make it into the secular document which now governs India, such as ideas of Ahiṃsā and fundamental freedoms, though likely Ambedkar would resist any comparisons due to his hatred of the document.

The Manusmṛti continued to be an important document well into the 20th century as a philisophical and ethical text and remained (and remains) in wide circulation. Scholars tended to praise passages on equality and Ahiṃsā while decrying those on Varna and Jati. The document continued to influence Indian law throughout the 20th century.