Why do I need an ID for this all age concert?

It's at a club but the shut down all the bars for all age concerts. This is hollywood undead I already Bought tix for me and 2 other people print at home. I've got the reciet and everything. I emailed them if i need to cut them out they said no bring the whole thing. And the past couple of times the keep saying i need an ID for an all age show with like 900 people that go. Why though? I'm only 14 in homeschool I cant get a damn ID.