Let's be honest with ourselves, none of us is perfect in everything. I mean, I'm a programmer, and a, well let's say not the worst, 2D-Designer. However there simply are things I'm not good enough to create myself. So, in order to still create good looking games, you need someone who can do this for you, and thanks to the AssetStore, that's pretty cheap nowadays. Also there are enough situations where you just go: I could work 50 hours creating this, or I'll just spend 20$ for pretty much exactly what I want. Well, what you gonna do?
Over the last Year I bought quite a number of Assets there, so here's my top 5! I'll focus on "programming" stuff, because these are things you need in most projects. I bought a lot of models too, but these where pretty "project specific".