The front fills are Q cabs and can throw pretty damn well. They are on a separate aux so you can put only things that you want in them versus the main PA. This was the case with the EAWs too. Reason is that you don't want to put the same mix there as the stage throw mucks it up and you don't want to stress the Qs (or those cute little EAWs that were there before) with bass and kick. Problem is that the infills can also add to feedback issues as they do spill onto the stage (not as much now though).
I still remember the story years back when Helmet played, and Paige asked how it sounded. Someone right up front said they couldn't hear the vocals. He replied "Well get where the speakers are".
Usually the headliner will have a guy and the openers don't, so that is the club guy who has lots of experience with the rig mixing. Headliners usually have great guys, but sometimes they have sub-par guys or they make it sound like what the egos in the band want (and not what should be). And remember, as taught to me by one great engineer, "Sometimes you just have to turn it up".