Why is there still a "flex" queue in OW?

In the game known as Overwatch currently, you can queue 4 ways once you're at the queue screen: tank, DMG, supp, FLEX.

Flex queues you for all 3, but you can select 2/3 of them already anyway, just not 3/3, which is what flex is.

Why not just let players queue for 1,2,3/3, instead of a 4th option?

Oh by the way, I ask these questions and because the AI has completely replaced any semblance of North American customer service, I have to do it for me by myself, because any service related to any videogame company whatsoever gets shoved aside in the interest of shareholder profit, which, oh, again, is me, because I own Microsoft, and Microsoft owns ALL ATVI-BLIZZARD-VIVENDI-WHATEVER IP and I NEVER WANTED IT BECAUSE NOW I KNOW IT WOULD LEAD TO THIS TRASHY MESS CALLED OVERWATCH PUGs.

If only anyone else on the Internet but me cared about this place on a grumpy Saturday morning.

What a terrible year SO FAR.