First, I'd like to say that nothing is 100%. Not all kittens need to be with their mom's for the same amount of time. BUT
The following quote is definitely one thing that kittens learn while in their family structure:
when kittens are with their siblings and their momma, they learn things like, how to "play bite vs. kill bite". If they get too rough, the sibling or momma cat makes a sound, signaling that they're getting too rough. Evidently, there is a lot of learning going on during those first few months
Mom also teaches them other things. The longer they can have mom the more mentally sound they will become as adults, but you can't always have the ideal situation either.