My thought would be that it lets you play with friends to ensure you are playing with a teammate you actually like or to allow you to try unconventional duo lanes or champion pairs. I think the first part of that will be very important to me if I ever finish my promos, since the three ranked games I have played have been the three most toxic games I have played since I made my account. I would like to play with serious teammates against serious players to improve myself, but I think it would be all but unbearable to play ranked without someone I actually like queueing with me, even if we don't wind up with a duo lane together. Sure, it can be a pain to play against a good pair, but a lot of the self-proclaimed duos I see suck really hard, so I imagine it evens out in the end.