Why would I go Fill

"Fill" does not mean "Random". It means "fastest possible match", which means you get the highest priority role, and that tends to be Support. If you don't want to fill the highest priority role I don't recommend selecting the option for it. If you want random roles I suggest rotating the roles you queue for.

Edit: Oh ya, like NetkingZetto said, next season they plan to allow you to opt out of a role you don't want to fill into at all. This will make queue times slightly higher because many will veto Support, but you'll never have to play the one you don't want.

Rito is adding in an option to exclude a role from your options when you go fill. There's your reason.

I got mid top and support for filling 3 games