Duo players must choose three roles?

The title is pretty much self-explanatory. I'm not sure why, but when I duo up with someone for bot lane or jungle/top, we still have to choose three roles. I'm sure there's a reason for this, but I did some searching around on the boards and couldn't find anything to explain it. If we queue up for three roles, there's a chance we may not get the roles that help us best when we duo. Can anyone explain to me why this works better than getting the placements we want when we're not more than two? Any help is appreciated, thanks everyone!