A company called Livenation spent the last few years going around buying all the venues that were big enough to hold large acts. They then merged with Ticketmaster to form one big monopoly.

This role is usually filled by the promoter. The promoter model actually is fairly smart, they maintain local offices in each areas and are responsible for knowing if a show will work in a region, and then booking it accordingly and taking care of advertising and PR.

Now then, of course, companies like LN are forming that are promoter/venue/manager combos and it gets a bit iffy.