And this isn't about 0 offers vs. many, it's 1 offer vs. many.
Nor does it really protect you anyways, if you accept one of those offers and then reject the others after your starting date, but the one you picked lets you go the next day you'll still be in a very weakened negotiating position again when you come back for a do-over on the other offers.
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.
Same philosophy on the employee side, not beneficial to ride everything on only one offer.