For us, I sent DD1 to preschool at 3 because I was overwhelmed by having newborn twins and a serious chronic illness, and unable to leave the house very often, and she was going stir-crazy hanging around the house. Preschool gave her something to do that she didn't have to share with the babies, gave me a chance to zero in on the twins' needs, and gave her a chance to interact with other kids, which she wanted badly. She was naturally not outgoing, but WANTED to learn how to socialize, and I was just too busy and crazy and overwhelmed to help her.
I won't, however, send the twins so young. I won't have a baby tying me down, so when they're old enough to have a serious interest in interacting with other kids, we'll just find free or low-cost opportunities to get them some friends. Preschool costs a fortune around here, and I want to keep them close to me as long as I can. I might send them twice a week when they're four, if they seem interested in stepping out into the world a little. I think it might be a good opportunity for them to try going out into the world a little without me. But I might not; I don't know yet.
I think though that if you're in a neighborhood where most kids go to preschool, it's really hard to find kids who AREN'T in preschool, so if your child really wants to play with other kids, it can be hard to find them. I also think that a lot of parents have given in to the pressure of thinking their kids need academic preparation so early.
I think preschool is great, as long as it's a fun, low-pressure, child-centered environment. I also think no preschool is fine.