Others have linked it to you but let me tell you why I like it. It lets you generate a new email alias and password instantly whenever you make a new online account somewhere. Or just whenever you want. I've been slowly changing all my accounts over to their own unique email alias that can't be tied back to my main email. My main address is known by nobody at all.

The main benefits are if someone steals a password, the email address that comes with it will only be useful for that one account. (I don't need to go over the benefits of a standard password manager.) and so if that email is leaked or added to a spam list, I simply delete that address after changing the address for the single account it was used for. I can tell exactly which address is getting spam easily. 0 spam. Ever. Spam email has been solved for me.

Proton remembers which sites use which email/password as well.

Other than that, it's just good for privacy. Having a different email for each account makes it harder to track a user across accounts.

These addresses are somewhat auto generated, with the name of the site along with a random word and a few numbers. But if you want to create another email address, you get a handful of custom ones for free with the subscription too. You can revoke these the same way, so you can have a professional looking email to hand out to people that's not auto generated, without giving out your account's root email address.

Edit: I also want to specify that while all of this is technically possible through other means, Proton makes it easier than any other option. Plus access to a good vpn, a nice replacement for Google drive (for storage and basic editing, at least) in addition to the email service and password manager mentioned above. A very good deal, in my opinion.

Edit 2: it sure sounds like I'm a paid shill but I can assure you I just really fucking love Proton and I get too excited about things.