Some competitions on Wattpad are run by Wattpad themselves (E.g. the annual Watty Awards), while many others are run by writers themselves.
Most competitions are published as Wattpad books where you can comment key details about the work to nominate yourself.
Nominating your stories for awards could prove a huge positive.
It's essentially promoting your work, even if you don't win.
But, it depends on the scale of the competition and therefore the audience that are likely to read it, if it wins.
However, I'd say 'why not?' to entering, especially smaller-scale competitions.
It doesn't take long to write a nomination and just like every other comment you post, it still places a link to your profile into the community.
In terms of larger-scale competitions like the Watty's, it's probably still worth entering if you have a finished work that you're happy with.
But, still be realistic about the prospects of winning.
Do you enter Wattpad Writing Competitions?
I can certainly see the appeal in having a competition winner sticker to put on a story cover.