In general, contests can be useful and not just for photographers.
Besides the bragging rights it can be used as the proverbial carrot not the stick.
I stopped entering them with the hopes of winning once I was hired full-time as a photographer at the newspaper.
It didn’t mean I was resting on my laurels.
I still worked at my craft but I didn’t feel crushed if I didn’t win.
These days I use contests as a way to get me out and about to shoot with my camera.
To win any photo contest often requires lots of luck and some skill.
Most of the time, it depends on the pool of entries.
To some extent if there are no categories, then you may have a chance to stand out simply because your subject.
I recently placed 3rd in Cambridge in Colour’s August 2014 (monthly) photo contest.
It’s a nice nod especially since I haven’t seen a lot of pictures with people win in the past.
Generally speaking the winning images on that website have been mostly scenics.
My previous win at Cambridge in Colour’s monthly contest was a 2nd in Sept 2013.
There are no prizes in winning or placing that’s why I enter these contests.
Perhaps that’s the reason I placed, it’s not as competitive.
What the reason you believe, remember to read the fine print and don’t be a victim of a rights grab photo contest.