It makes sure that when you type www.somewebsite.com and press enter you go directly to https instead of going through a http-https redirection that could be intercepted. This is only useful for websites that support https but are not in the HSTS preload list. Alternatively you can enable that Firefox setting someone mentioned but once you do that you stop being able to visit websites that don't support https so HTTPS Everywhere is a good middle ground.

Firefox now has HTTPS requirement as an option in settings.