I use it for more than just email. I use their email, VPN, cloud storage (Proton drive) and password manager. They have a calendar too but I don't really use that.
They are very privacy focused, don't keep logs and are based outside of the nine eyes countries (not that I'm too bothered by it but some people want that). Unlike Gmail and a lot of other providers they aren't interested in using you or your data to sell advertising or sell to others. If your email provider is free then they are selling your data.