I must say that I personally did not see a need for preschool and my child was the one pushing for it at TWO. Other parents around us thought NOT going to preschool was tantamount to child abuse. I couldn't figure what it was for since he had been reading for some time and seemed to do well socially. Our mom group had a meeting with reps from several local schools and once my child got a taste he was set on going and so we did.
Where we live there are VERY different preschools with very different philosophies. I avoided the structured "teach them about school and academics" ones and found a play-based one that my son enjoyed thoroughly.
I learned about him through the eyes of teachers and began to see the different view from those who "got" him and those who did not. It laid the groundwork (for advocating for him) that has been very handy as we've moved along in to K.
I also realized how bad Kindy would be by seeing how the shift in to learning phonics affected him. It set in motion our testing and reading here and our search for adequate school environments. (Which by the way landed us in a stellar gifted JrK where he got to play with simple machines, discover real math, and spend time with other gifted kids)
I got to see how kind and well meaning teachers who really cared for him were unable to meet him where he was no matter how they wanted to. His 4yr old teacher told me that in 10 yrs she had never taught another child like him and searched for ways to make the classroom work for him. But she was to teach phonics and writing letters and he could neither tollerate the phonics nor write the letters.
We would have so much less understanding of our child without what we learned in preschool. We made good friends and charted our path. In the end it was 3yrs of preschool before K and I'm thankful for all of it.