Some people don’t care about having an unique identity and actually favor creating multiple accounts on each service, to present themselves with different avatars depending on who they are interacting with. They are not “attached” to their identities and see this an opportunity to stay pseudonymous online and protect their “real” identity.
Some people think that the instance you join should be also somewhat indicative of your tribe and that they should be able to filter out who they talk about by checking the domain. This view is especially favored by the Mastodon crowd.
And then some other people (I think I would include myself) would like to be able to not just “use” a single identity, but to have portable identity in the Fediverse as a way to ensure that we can remain sovereign over our online presence. I would personally love for Communick customers to be able to use their personal domain, because that would mean that if even if I closed down things tomorrow, they would be able to migrate easily and without depending on me.