I don't understand the question: Gmail has one address and it is not on your domain? Can you elaborate please?

Simple use case: you've got several startup projects with different domains, and multiple addresses on it. Migadu makes that painless. Family domain with emails for the whole family? Easy.

And..of course there are many alternatives. No intention to reinvent email here. Just making things more pleasurable to manage and use, by getting them out of the way.