For me, its about invasive weeds, mosquitoes, ticks and other non-desireable bugs as well as rodents. But I live near a field! So the lawn serves as a buffer and lawns are very good at suppressing weeds. I could go with clover though which we are cultivating on the small hill next to us to buffer us from the field instead of grass though.

A lot of my neighbors have had their lawns overridden by crab grass anyway so is that still a lawn at that point?

Having a lawn also is nice for the kids.