Power
If power was water, then ushers would be somewhere near the arid Ahaggar Mountain region of the Western Sahara desert. If your job is guiding people to their seats you, predictably, have control over just two insignificant things: whether or not you guide people to their seats and, in rare occasions, whether or not they can stay in those seats.
For example, if a person attempts to enter a theater or auditorium without a ticket or, heaven forbid, with outside food or drink, it's an usher's job to flex those figurative muscles and tell that punk in no uncertain terms that he is not welcome in these here parts. Or, you know, that he simply needs to finish that Big Gulp before coming inside.
You may also find yourself making judgment calls on who stays and who goes. Is there a disruptive row of teenagers seated near the front of a quiet movie? It's your job to round those little miscreants up and kick 'em right on out to the sidewalk.
Trust us, you haven't lived until you've had to calmly explain to a teenager's irate mother why her daughter was asked to leave Legally Blonde 4 thirty minutes before the credits. Talk about a rush.