Not to steal @James Kerman's biscuit but he's a moderator so he may not always have time, you can always message me!
1. KSP is very realistic, and KSP can really teach you something about orbital mechanics!
2. It is very, must I say very, rewarding when you get into orbit for the first time/land on the Mun/land on rover on Duna, and you and your friends could have something to talk about when you all meet up!
3. Replications-although you can always make your own rockets, KSP has some special parts to replicate real life missions, and the thrill of doing exactly what happened in real life!
I could go on, but these are some good things to tell your friends!