When I started writing many, many moons ago, a friend told me to enter contests even if I had no hope of winning. That seemed like a strange form of masochism at the time, but over the years I've come to appreciate her advice for many reasons.
1 - It's a free, or low cost, way to get objective critiques of your book.
2 - It makes you feel like a part of the writing community.
3 - There's always a deadline and deadlines are good.
And today I discovered a fourth: when you've been through a long period of agent rejections, it feels really good to be told that even though you didn't win, you made it to the finals and the reviewers think the book is worthy of publishing.
Thank you, reviewers.
PS. I did win one!