Because the seats on aircraft are fucking torture devices, and the sooner I can stand up the less pain and discomfort I'm in.

I stand up when we land sometimes to stretch my legs, not to hurry out. Generally I sit back down after a minute or two, because they feel better.

I've always assumed that they eventually realize they've misjudged the time to stand up, but most decide to commit like they meant to do it instead of sitting back down.

I'm a large man at 6' 2" with large shoulders. I am crammed into the small economy seats trying not to spill over into the seat next to me (most often occupied by my wife) or the aisle. God forbid the person in front of me lean back their seat. I've tried wear knee pads to save my knees. At the end of any flight over two hours, I just need to stretch my legs. I get up, but I'm not trying to get off the plane. I'm just trying to de-acordian myself.

I typically try and get front row on flights, or at least the row next to an actual exit and I only take hand luggage. If I can get my hand luggage and get off first I get to passport control and get out much quicker than people who wait and end up at the back of the passport queue, and even further ahead of those that do baggage claim.

I can easy save 30 minutes if not more doing this. Obviously its not going to work every flight but it works more often than not. It's particularly worthwhile if you catching a taxi at the other end as you can short cut the queue at smaller airports.

However I don't see the point if you have way down the plane and/or have checked bags, other than leg cramps.