What does Sitemaps really do anyways?

My 3 sites started off as duplicates of each other. Through multiple
emails, I've made changes and now pass copyscapes duplicate content
filter.

http://www.tennisdc.com

http://www.tennisnortheast.com

http://www.tennisphilly.com

I've resubmitted the sitemaps to google multiple times, but it seems
like everytime they download, they don't update any information.

So what does sitemaps even do?

-Steve