Scope reflects a bit of a relic of the development history of the application; Self Service being originally a separate application from the HEAT foundation (object workspace view). That's less relevant at this point, but the command is still necessary to direct the application to successfully apply any subsequent commands, like workspace, searches, UI actions, etc.
Scope is required, but Role is not. (For the most part. You CAN leave Scope off, and only define the Role as Self Service, and it will successfully switch your Role, but it won't take you to the workspace defined, run any searches, etc). If you use the Self Service scope and leave the Role off, the workspace will still open as long as the current role has it (e.g. the Service Catalog or My Items). If the role doesn't have the workspace, it will just open to the Home page or the last cached tab. (e.g. if the default Admin role tries to access a URL with a Scope of Self Service that opens the Knowledge Center but does not define the Role as Self Service). So using Scope but not Role will not switch the Role of the current user, it will just take you to the Home page, and using Role without Scope will switch the user's Role, but will just take them to the Home page. It's best to use both. If you have notifications that go to specific users that aren't likely to be changing between Roles, then the Role command may not be necessary, but the Scope always is.