The hub (admin UI) itself and the Hubitat Portal are two separate logins. The hub can only have one login, effectively an administrator, or it can have no login at all (open access) if you leave it as it is by default or later remove the local user you created.
What you're looking at above is the Hubitat Portal login. An "admin" here can do the fully gamut of things, like adding/removing other (including guest) users. Hubitat Portal logins can be used to sign into the Hubitat mobile app (not the hub admin UI). One reason you might want to add a "guest" account here is for presence--someone else in your house could add the app, sign in to the app with their guest account, and create presence device on your hub for their phone, but they won't be able to log into the Hubitat Portal website and, say, add other users. (Note that you can't actually create an account here; as the UI says, it has to be an account that was already created. They could do that for themselves, or you could do it for them.)
I'm pretty sure they'll still have access to that hub's Dashboard apps, or at least this post from shortly after the introduction of the app (I don't recall any big changes since then) says that the above is pretty much the only difference:
If restricting Dashboard access is a concern, the PIN feature mentioned above is one option; otherwise if you don't need the app for other reasons (like presence), you could simply share the local or cloud link with them and disable the options for that Dashboard that would allow you to navigate back/up and see a list of all Dashboards for that hub. If you have editing features enabled for that dashboard, a guest would still be able to do that (but a hub admin could go to Apps > Hubitat Dashboard and change any of these settings).
