Preschool

What do kids learn in pre school?

My kid is starting to become preschool age and I have been starting to think about if it's needed or not.

I worry it will be stuff he already knows. He just turned 3 recently. And he can count to 20, knows the colors, knows the alphabet, can identify a bunch of shapes. I of course don't want him to be behind, so any information would be super helpful! Thanks you!!