You may be lucky enough to win, and that's great. But let's say you don't win (like most people) - what have you just agreed to? As an example here's an excerpt from the Smithsonian Magazine's latest photo contest:
"By entering the contest, entrants grant the Smithsonian Institution a royalty-free, world-wide, perpetual, non-exclusive license to display, distribute, reproduce, and create derivative works of the entries, in whole or in part, in any media now existing or subsequently developed, for any educational, promotional, publicity, exhibition, archival, scholarly, and all other standard Smithsonian purposes. Any photograph reproduced will include a photographer credit as feasible. The Smithsonian Institution will not be required to pay any additional consideration or seek any additional approval in connection with such uses."
Sooooo ... for absolutely no compensation you are giving Smithsonian the right to use your image(s) in any way they want ... forever. Did I mention that you get not a dime from them for that right?
For some contests your entries are limited in size, which is a good thing because it limits the potential usefulness of your image by those running the contest. But the Smithsonian has no resolution requirement:
"Digital photographs should be taken at the highest resolution possible."
Oh, and maybe you're thinking they severely limit how many images you can enter. Nope:
"You may enter up to seven (7) photographs in each of the five categories with a total contest submission of thirty-five (35) photographs."
Thirty Five is a LOT of photographs. And you're just handing over hi-rez copies to a magazine to use any way it sees fit, forever, with no compensation.
Still interested in entering that contest? I'm not.