Champions That Aren't Supports Played As Supports?

Hey all. So I have a question - I imagine it sounds silly to most experienced players, but this is something that really confuses me. Why do people play champions that aren't supports as supports? It doesn't really make sense to me. I kind of sort of understand and , as they both have a stun...But why in the world would you play an AP-based champion as a support? Pretty much all you've got is a stun. I haven't experienced this before but I have heard that is played as a support more often than mid, his intended role. ...What? Why?

I know I sound quite ignorant here to people with experience but this really baffles me. I initially thought it was something only skilled players use, but then I get what happened to me last week where tons of people were playing (who was free at the time) support. I really don't understand this at all; can someone fill a confused new(ish) summoner in on why playing champions that aren't supports as supports is a thing?