You want to have multiple offers because it helps you leverage yourself to higher pay during negotiations, and it also gives you more choices to decide where to go next.

You want to have a offer before giving notice just so you know what's next, and obviously you shouldn't do this if you don't intend on accepting the only offer you have.

If you have no offers you can quit, but you have no guarantee of finding a job immediately and it also can hurt your leverage while negotiating.