Comment by cosmic_cheese

Owning the domain your email address uses gives you a greater degree of ownership over that email address and makes you service provider agnostic.

Using an @gmail.com address for example, if you decide to move to another service provider at some point or especially if your Google account gets banned, you’re stuck manually migrating over however many things you have attached to your address (some of which may not be easy or possible without access to the original address).

In contrast, if your address is on a domain you own, the provider becomes moot. It doesn’t matter if you migrate or get banned, you still have your email address, and after a small blip between providers all is as it was.