"Yo, my man!" This exclamation is used to get somebody's attention. Sounds contemporary, but surprisingly, "yo" goes back to the Late Middle English of 1400. Soldiers in World War II used it to mean "present" in a roll call. Sylvester Stallone, in the Rocky movies, used it a lot, too. "Yo, Adrian!"
In Scrabble, it's a clever way to dump a pesky "y." It may only score you five points, but that can make a difference in a tight game.