What's the point of using a custom domain

The email domain is the part after your name and the @ symbol. It generally reflects the name of your service provider. For example, your standard (free) iCloud email might be droppindimes@icloud.com, but you could change it to a custom domain to make it more like droppindimes@coinmaster.com (or nearly anything else, really).

It allows you to personalize your email, making it suitable for business and other professional endeavors. The domain can also be used in-family with Family Sharing.

Here's an overview - Use Custom Email Domain with iCloud Mail - Apple Support

Some extra articles-

Set up a custom email domain for iCloud Mail on all your devices - Apple Support

Add a custom email domain for iCloud Mail on iCloud.com - Apple Support

Personalize iCloud Mail with a custom email domain and share with family - Apple Support