How do contests like CodeChef help you as a professional?

I'm still in high school, so I this question keeps popping into my mind.

How do contests like CodeChef, TopCoder, USACO, etc., help with your day-to-day job as a programmer?

Do you use the same algorithms/techniques while doing professional work?

And, do you recommend high school kids participating in such contests?

I hope these questions aren't too vague. If they're confusing, I'm very willing to clarify them in the comments. These questions have been bothering me for a while.