Xavier, you have it exactly backwards. It is kids from poor or dysfunctional homes who most benefit from preschool. A good preschool will try to replicate the enriched home environment of more fortunate children. Children are read to, and taught to cooperate and share with each other. Often, they are taught beginning reading and math skills. They are taken on field trips to places like zoos and children's museums.
And I think there are benefits not even tracked. Parents dropping off and picking up children see what an enriched environment looks like. They can discuss child-raising issues with the teachers. Their other children may benefit from this exposure. Maybe most importantly, the parents can hold down a job without worrying that their child is stuck in front of a TV all day by a careless babysitter.