You'd be very surprised how many websites you visit in a day that still don't have HTTPS, if you stopped to check. Little academic sites, old tech blogs, those trackers when you click a link in an email list that you signed up for...
If a site doesn't support HTTPS the extension won't do anything.
It has an option to block sites that don't do https (with options to permanently/temporarily allow them.
I have some config (afaik found it on ghacks sometime ago) in Firefox that breaks the entire website if it has any HTTP url in it. I rarely see broken websites.