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.
I use it to block HTTP by default.
On firefox you can do that anyway by setting dom.security.https_only_mode to true in about:config.
No need for an additional addon unless you need to handle specific exceptions.
Firefox can do this with some config currenty.
Of course it's needed, regardless of the existing coverage from existing websites. HTTPS provides confidentiality but also integrity and may be used to provide authenticity.
The addon is entirely related to existing coverage of websites because all it does is replace http urls by https ones if they exists. It doesn't magically add https to sites that don't support it.
All those people who downvoted me don't understand neither HTTPS nor how this addon works. The addon is only at value when you request an HTTP url for a website that doesn't automatically redirect you to the HTTPS url. It's almost useless since the vast majority of websites automatically redirect you.
I thought the extension sends you directly to the https version, that is similar to how HSTS works, and securitywise alot better that http redirects.
Of course it doesn't magically add https. I never said it did. As you said it enforces use of https instead of http. That is what I said was necessary. The three reasons that is necessary were what I listed.
It doesn't yet, but theoretically it could redirect http URLs to archive.org/archive.is, which have https.