why do they put vaseline on boxers faces

Many people in the mixed martial arts and cutmen business remember БGrease GateБ and the changes that came thereafter. БGrease GateБ happened during the Georges St. Pierre versus BJ Penn 2 fight at UFC 94.

During the course of the fight, GSP s cornerman, Phil Nurse, applied Vaseline to Georges face. He then touched GSP s shoulders and back before wiping his hands, thus accidentally transferring Vaseline to GSP s back, which, according to his oppnent BJ Penn, made him more slippery.

BJ filed a complaint with the Nevada State Athletic Commission (NSAC). The victory was subsequently upheld, but as a result of the controversy, new rules were formed by the NSAC and UFC cutmen including Jacob Stitch Duran.

The rule changes state that only neutral cutmen may apply vaseline to a fighter.
It's actually a chemical (adrenaline 1:1000) that you'll see the cutman apply with a cotton swab that coagulates the blood.

The vaseline gives a bit of protection for the cut, so if you get hit again in the same place hopefully it won't get worse.

You put it on the your face before fights/between rounds (and in sparring some coaches like to put it on the gloves and headgear too) becase it will help the punches slip off the skin to reduce the chance of cuts or bruises.