Namespaces help different projects, teams, or customers to share a kubernetes cluster. First, they provide a scope for Names. Second, as our access control code develops, it is expected that it will be convenient to attach authorization and other policy to namespaces.
Use of multiple namespaces is optional. For small teams, they may not be needed.
Namespaces are still under development. For now, the best documentation is the Namespaces Design Document.