There are a few writing contests that garner major attention for the winners (for example, from agents, publishers, or producers) but for most competitions, the reward is the ego boost of winning and a sum of money. When deciding whether to enter a contest, consider whether or not winning the contest would carry any meaningful prestige, the value of the prizes, and the amount of the entry fee.
This subject came to mind when I got an email about the Spring Short Story Contest sponsored by The Writer Magazine. The top prize is $1000 and publication in the magazine, the second-place prize is $500 and publication in the magazine, and the third-place prize is $250 and publication on the magazine's web site.
These are good prize amounts, but the entry fee is $25 for your first entry and $15 for any additional entries. With a prize total of $1750, they need only 70 entries @ $25 to cover the prize money. Yes, they do have to pay someone to read the entries but these are short stories with a maximum of 2000 words.
You can also get 200 words of feedback on the story for an additional $25, which is reasonable. I don't know the quality of the feedback on offer, but especially if you don't have any other way of getting feedback on your writing, it may well be worth paying the extra amount.
They say, "Our critique times vary based on how many submissions we received and can take six months after the contest has closed." That would suggest a large number of entries.
The magazine also ends up with a nice mailing list of contest losers who can be pitched courses and subscriptions. The winning writers will appreciate being able to show their friends and families their work in print--however, it's not bad for the magazine, either, since they will be filling a number of pages without any additional payment to the authors.
I'm not saying you shouldn't enter this contest. If $25 is not a meaningful amount to you, that's fine. However, if money is tight, contest entry fees can quickly add up and your odds of winning are fairly small.