We all know that standing on a hill feels better than standing on a flat.
Even pros seemingly only stand up for hills (not counting sprints).
What causes this?
Here's what I've thought of so far:
Often you can ride up a hill without shifting out of the gear you were using before. If you were to stay seated, you would need to shift down.
For a long time (road) bikes had such high gears that you had to stand for many hills. That's not the case anymore though, so maybe it's a moot point.
When you're on a hill, the bike is tilted backwards. This makes it easier and more natural to stand up, because the bar is "higher" and more reachable than it would be otherwise.
But I have a feeling there's more to it than this. These reasons are not significant enough to explain the difference I can feel, or the way pros behave.