If your business is good or popular you hire doorkeepers to make sure only the right people get in. If your establishment is bad, new, in high competition, or of low interest to potential customers, you hire a different kind of doorkeeper to encourage people to come in. It's the same kind of goal, but different demand makes for different approaches.
Mokalus of Borg
PS - Most websites are in the second category.
PPS - And the "doorkeepers" in that case are advertisements.