I would guess to prevent professionals from taking pictures “for free” instead of getting contracts with the bands/venues and paying fees for the privilege.
It varies. A couple of times I just sent an email and they told me when/where to pick it up. Other times they wanted to see my work - a few of these, a personal blog with basically no traffic was enough to pass muster, a few, they wanted to see me working at some sort of actual publication (digital was fine, but it needed to be more than "cthalupa's concert blog")