What exactly is the point of having team 5v5s

The point is fair play, it is completely unfair for a full 5 man team with proper communication with each other to beable to play against players who might be playing solo or with only 1 friend.

Teams have stricter timeframes in play; a 5man does not. You could, given enough friends, be in a different team of 5 every night. Whereas trying to manage that through official teams would be a nightmare.

What Riot probably hopes will happen (and for once this is actually a good idea) is that you have a good game in solo q, then whatever group is in that game picks you up. You may or may not ever play with them again, but for now you have found a group of people who function, which is a lot better than the constant roulette of shitbirds and feeders in current yolo q.

Obviously work must be done to prevent mismatched teams (5 should only EVER play 5, 4+1 should only EVER play 4+1, etc) and the matchmaker could use a complete overhaul to start with, but the general idea is sound.