It does serve a purpose, though, it's parkland ultimately. People use their lawns to host events that might be larger than what their home could host, they are a safe area for children to play unsupervised, it's an area where you can produce food for yourself without having to spend earned income to feed yourself, livestock and pets can roam it, etc. Plus, many cities in the U.S. are park poor and the only greenspace really is what is available privately.

Those points are mainly true of back yards, not really front yards though.

It depends on the local regulatory environment. You can do these things in your front yard where I live (based on what I've seen among my neighbors at least).