What's the point of priority boarding an aircraft? You don't get to your destination any quicker.
Priority disembarking would be much more advantageous.
Well there are a couple of benefits.
First, it means you get to the overhead lockers when there is still space in them. Second, it means you get to sit down rather than standing in a queue.
And (more rarely) if there aren't allocated seats, you get to choose one which gets you near an exit for disembarking.
Still not worth paying for IMO, though some obviously find it is.
What I DO now normally choose if available is fast track security.