One of the local open mics is hosted by a bar during the slow season. Then they reward the good acts by giving then paid slots during their prime season. This has a couple of perks:

1) it helps them draw customers during a period that would have been slow otherwise

2) it lets them know which performers are good so they can book acts based on real knowledge of their abilty to perform and draw a crowd

3) if they make the open mic a pre-req for paid gigs, it raises the bar for bands that will play at the open mic.

(Around here, open mic events tend to be full of performers who aren't good enough to play for pay. Therefore, the open mics don't draw much "audience" beyond the performers and their friends)