Personal space, not being around family all the time/when you want to make noise (if you get my drift). The space to start a family and the right to do whatever the hell you want to do in your house, like renovating. Whatever you say, you do not have 100% independence and freedom when living with your folks, unless they live in an urn.
At least for myself, I'd be pretty conscious of the fact my family were in the house if I was having private time with my girlfriend. Rent hasn't been cheap but I am grateful for the space and independence I have. That said, it's not for everyone, and staying with folks is a great way to save up for a deposit for your own place.