once you have a domain you now have the ability to own stuff online. that is, independent of any corporation. The main benefit this gives your average person is the ability to move email around between server providers. but you can now have a web presence that is not (facebook, twitter, hackernews). nothing against sites, they are fine, but it is nice to have an online existance that is independent of them.
Unfortunately the bar is a bit high for most to realize most of this. I miss the days when your isp would offer web, ftp and email hosting.
It's good for having your own website. That's super easy to host and use.
It's quite another thing for email. While it might not be that difficult to set up a basic email server, but to actually get it set up all correctly, and secure, and whitelisted, and get anything else on the 'net to actually interact with it is a little less trivial. And an email server that can't effectively send email (that won't be ditched along the way by some anti-spam measure somewhere) would be pretty limited.
Maybe useful for receive-only stuff like password reset links or one-time authentication links though?
You can host your email with your own domain with one of several services that take care of all the details (mailgun, fastmail, protonmail, etc), If you have problems with the provider you change your domain configuration to a different provider. Does ot solve all the issues but it is more flexible.
You don't have to self-host it, you can point it to an email provider (Gmail, Fastmail, Zoho...) and let them do the hosting for you. If they ban you (like Google likes to do), just point to another service and keep your email address (and therefore access to other services).
I work as a sysadmin and I don't want to bother with self-hosting my own email. I happily pay someone else to do that for me.
Unfortunately the bar is a bit high for most to realize most of this. I miss the days when your isp would offer web, ftp and email hosting.