The extension makes you use the HTTPS connection when the site you are connecting to is known to support HTTPS.

Websites can automatically redirect to HTTPS if the client connects on http, but many websites don't redirect

It has the option to block HTTP traffic, making sites that don't support HTTPS unusable.

You could create a separate "secure" profile and feel safe that all traffic is secured, while still being able to browse HTTP in another profile, for instance.