Precepts may have many different purposes:
Keep people in line. (May be helpful in a society and in a spiritual community.)
Mimic and get somewhat used to how it is to live from more clarity. (May make the transition a bit easier.)
Stay out of trouble so there is more time and focus for practice. (Engaging in what the precepts aim to keep us out of tends to create messy situations.)
And the one I find most interesting for myself: The precepts helps me notice when I don’t follow them, or when there is an impulse to not follow them. I can then see if there is a belief behind that impulse, identify it and take it to inquiry. For instance, is there a belief/fear behind an impulse to lie, even if it is “just” a white lie? What do I fear may happen if I tell the truth?