It's called "butts on barstools"
As a band, it's your job to bring people into the venue who are going to stay and listen to your music and while they're there, they'll be shelling out money for alcohol. Your value to the club owner is determined by how much alcohol gets sold while your playing your music versus how much would have been sold had you not been there.
Hence, if you want to be paid, you need to convince the venue that you can bring people. It really is as simple as that.
Couldn't have been articulated any better.
Last one we did, we pulled 96 people for a 10.00 cover. Not bad, but it wasn't easy.