It's a bit like the Proton version of Checker Plus for Gmail, but with the premium subscription of being able to host and access multiple Proton accounts altogether at once. Like with Gmail's Checker Plus, you do get several benefits like email notifications per account simultaneously which is always convenient to have.

EDIT 1: It's changed into a 3 star for me because the login session expired after a few days and you're unable to relogin unless you have premium subscription. Even though access to 1 account was supposed to be part of the free tier, the login is apparently still locked behind paywall regardless.

EDIT 2: Although I'm still receiving that persistent "login session expired" prompt every once in a few days, I was able to successfully relogin into the Proton Mail Checker this time around without the paywall issue getting in the way. Though I kinda wish that I didn't need to keep logging back in again and again all the time whenever that prompt returns. With this update, it's a 4/5 for me now.