When we dropped my son off at his boarding school, I could not help but be impressed with the two young men in his dorm who have elected to serve as proctors or prefects. Why would an upper-classmen want to live with much younger students and devote his time and energy to help keep them on track? Are these students paid? Does being a prefect help on your college application? What motivates these students to take on this position? Are they just doing this out of the goodness of their hearts? If so, God bless them. Both prefects in my son’s dorm were extremely helpful during the move in process and it gives me great comfort to know one of them is living right across the hall from him.
Prefectship is a coveted leadership position and does add to the college app. I know of more than one case where the position went to one kid over another kid because K1 needed beefing in that area, but all those chosen were fit for the job. I also know of cases where those in that position were expelled just a few weeks shy of graduation although I’m sure they appeared lovely to new parents on move-in day.
At Choate, some seniors just want to get back into Mem House for another year.
At Groton every sixth former is a prefect in a dorm and in other areas as well. It prepares them to lead. Last year there were lots of sixth formers who wanted to be a prefect in the lower school dorms.
When I was a dorm parent, the prefects were carefully chosen and vetted by the dorm heads/faculty committee and in our dorm the job also came with nice perks: a large single room with a private bathroom and a refrigerator. 
Ours aren’t paid. The kids have to apply, be interviewed, and get voted on. Two reasons I hear most often from these kids are A) They had an awesome prefect/proctor when they were new, and want to repay the favor or B) They struggled to find their way when they were new and want to be there so other new kids can have an easier time. Yes, it looks good for college, and yes, they get 1st pick in room lottery (many of ours still live with roommates) but I’ve never known a proctor for whom that was the prime motivation–the perks don’t outweigh the responsibilities here.
In our case, prefects are upper formers who love their house enough to want to stay an extra year. A good majority of the population here apply to become a prefect.
“Prefectship is a coveted leadership position” is right. At Thacher, all seniors have leadership roles of one sort or another, but only a few are prefects (voted on by faculty/admin/advisers etc). They are the most sought-after leadership opportunities in the School. Prefects have a vital player-coach role in the health of the community.
As others said, it is a leadership position, and a very coveted one at that, and, no, they do not get paid.