Writing, or doing anything else competitively has it's merits. While I try to improve my writing every day, having a competition in mind gives that urge to improve a bit more weight.
If I'm participating in a competition why not try to give my bests. Why else even bother? It gives more incentive to put some extra effort to the writing.
It's also not just trying your best. Often times even that is not enough for the win. What matters is the feedback and learning opportunities you get from it.
Seeing what others write with the same exact goal as you have can be eye opening. Sure, with such creative challenge there are many ways to comply with the given topic. It might be hard to compare different submissions, but you still see more possibilities on how to execute the task.
The feedback is the best outcome. Not only from your own work, but you also see how others' work gets graded. Who and with what kind of text gets into the top. What was valued more by the popular vote versus what judges thought. There is a lot of lessons to be learned from such a simple competition. You just need to acknowledge them all.