HSTS is designed to address the fact that HTTP is the default protocol on the web. You might run a secure site that redirects everyone to HTTPS, but your users will just type in foo.com and that initial HTTP request is vulnerable to manipulation and SSL stripping attacks.
HSTS allows sites to advertise that they are HTTPS only. See the Chromium site page for more details.
However, it's difficult to test HSTS. Chrome's HSTS database stores only the hashes of sites (which may not have been the right choice, but I don't see sufficient motivation to change it at the moment) so it's hard to edit by hand. There are tools like craSH's Chrome-STS to help edit the database, but it's too hard for the average developer.
So I've added a debugging UI to Chrome to query, add and remove entries from the HSTS database. You have to type about:net-internals into the address bar and click the HSTS tab. You also need a version of Chrome after r75282. Today that means a trunk build but it should be in the dev channel release next week. Until then, here's a screenshot:
So, if you want to see if your site will break before deploying HSTS, you can add an entry locally for it and find out. Modifications persist on disk and have a expiry of 1000 days. You can also use it as a way to elect to always access sites via HTTPS (i.e. mail.google.com).