Putting forward serration on slides seems to be an increasingly popular custom modification to make press checks easier. How many of us do press checks?
To answer the obvious, "What's a press check?" questions:
To visually confirm whether a round is chambered, you bring the slide back just enough to peek inside, but not enough to eject the round.
PS. Dropping the mag and locking the slide back before handing over a pistol is more than just good manners.