Everything to do with one build. A project owns its budget, its documents, its contractors, its team, its phases — its own world.
Most homeowners only ever have one project. If you're managing more than one (a renovation and a separate rental, say), each one is its own workspace with its own email address and its own data.
Inviting your partner or your architect adds them to a specific project. Nothing leaks between projects unless you explicitly share it.