I mean why not. Having the Email provider automatically telling you that your E-Mail was found in a leak is really nice. Not everyone knows haveibeenpwned
Id need a paid haveibeenpwned subscription for domain level support. Proton is adding this to their existing subscription services to add value to what they offer. It's a smart move, especially once they add in catch-all/SL alias support
Looks like another Have I Been Pwned, at least at a glance they're looking for email addresses, and given that you have your email with them if you're using this service they kinda do have it already
I use my own domain, so I can do things like Home Depot, At my domain, and Amazon at my domain, and eBay at my domain, etc. So if anybody besides them emails me, I know that it has been misused or lost.
Some websites don't recognize the plus trick and won't accept it. And I use my own domain so that if I wish to switch email providers ever again, I don't have to switch all of my email addresses again. I can just switch my domain to a new provider and all of my emails stay the way they are.