Can probably get an offer is different from currently working at one of their competitors. It's a) a vote of confidence from another firm that this person is worth hiring b) an anchor point for your salary above where they would normally start the negotiations (yes even for "above average"). It removes all the guesswork of "can probable get an offer". You have one. Right now.

Additionally I've had negotiations stretch on for months. That isn't very comfortable with no income, and is an absolutely enourmous opportunity cost.