Your account is one identity, but your work may be spread across more than one workspace. Someone who looks after signage for two regions, or who helps several teams, does not need a separate login for each. One account reaches them all.
When you sign in, you are the same person everywhere. Moving from one workspace to another changes the context around you, the screens, content, and playlists you see, but not who you are. The account stays put; the workspace is what changes.
Put that together with its reverse, that a single workspace can hold many people, and the shape becomes clear: accounts and workspaces form a web, not a pair. A person can belong to many workspaces, and a workspace can hold many people. That is exactly why the two are kept separate. Tie a login to a single workspace and none of this would work.
What you are allowed to do can differ from one workspace to the next, because access is set inside each one. The same account might run everything in one workspace and only handle content in another.