Why do we need free will I don't think we need it per say- it probably depends on how you frame free will. if you mean the ability to choose between a number of options without that choice being pre-determined or 100% causal, it gives us a way to isolate blame and responsibility to an individual (or their soul if you believe in that).
hard determinists claim there is no free will, while compatiblists would reframe free will as more of a 'free necessity', where even though no alternate choices could be made, the freedom to affect and make that choice stems from the unencumbered mind (and not external forces/confusion/chemical imbalances)
even though the idea of free will allows us to dictate morality or blame, I think that treating our actions by the effects they have, as well as treating individuals by the risks they pose to the rest of us, still allows for the same level of legal accountability, just without labeling individuals as having 'good' or 'evil' intentions/actions.
so to sum it up: I don't believe we need free will, what we really seem to need is a way to justify restraint or consequence for individuals when their actions or risk are a danger to the rest of us.