As everyone has said it's about the overhead bin.

I don't use priority all the time, but if I'm travelling carry-on + personal item I will.

The other thing is on a long trip, sometimes if I've been in the terminal too long I just wanna move to somewhere else. Especially if I'm flying J class, then I know the seat waiting for me is gonna be fairly comfortable so it's nice to just get off your feet or away from the crowded gate and just hunker down.