Can someone explain spontaneous casting to me like I'm 5

So originally, I thought spontaneous casters functioned similarly to prepared casters, in that they set a specific spell into a slot they want to cast it from and are locked into that choice. But my friend is telling me that they basically function like 5e casters and can just cast any spell they want as long it's in their "repertoire" and they have the slots for it. To me that sounds really overpowered if that's the case, since there wouldn't really be a reason to play a prepared caster.