You use a @proton email address?, then that's basically seen as a gmail account. Nothing wrong with it, but it's just nicer to have an email with your own domain name.

Your nextcloud data could be accessed through your own domain name. Are you just using it locally?, or how you access it through the internet?, direct IP is not as nice as your own domain.

That's pretty much. You can use your own domain instead of either direct IP or a public service's name. It's just nicer, and I definitely notice when someone has their personal domain name.