To use the restroom, which, against all reason, is one floor down from my office. Because I work in a building old enough that back in the day they only bothered putting women's rooms on the floors secretaries work on, while they put men's rooms on every floor. Somehow, every science building I've worked in seems to have this issue.
But taking the elevator one floor? That ranks right up there in annoying-ness to:
1) People who stand on the street corner and ask "Do you have a minute to save the planet?" And I just want to shout at them, that's what I do every freaking day! And me giving you a dollar will not save the planet, because 90% of that dollar will go to pay bureaucrats and street hawkers like you.
2) People who don't replace the toilet paper/acid in the clean lab after using all of it (very different scenarios, but they truly feel about the same.)
3) People who take the bus one stop. Which is ~2 blocks. Or at least, I used to find this annoying, until I got pregnant. And started taking the elevator one floor.