For advertisers
If every slot on a space is taken, its sign-up page becomes a queue instead of a dead end. Place a bid — the minimum is always shown — and you won't be charged anything until a slot actually opens and you're at the top.
You can raise your bid or leave the queue anytime from that same page. Once you're promoted and paid, the slot is yours until you choose to stop; nobody can bid you out of a slot you're already running.