Its about being able to play the role you want to queue up for. If you ran 6 DPS against an opposing team that had a tank/support, you lose to just them being able to heal.

Someone has to pick some sort of sustain, or you die.

This means somebody doesn't get to play the role they want to play.

Additionally, the higher up in rank you got, the more important having an actual team composition got. In Bronze you could probably get by with 6 DPS, but the odds of this working drop down exponentially as your rank goes up.

Now if you know 2 heroes you're willing to play (in one role) you're guaranteed to be able to play something you wanted to play.