It is recommended by Catalyst that you keep an admin account and a user account with moderation privileges. Separating your working account from the administrative account ensures that making changes to the structure of your project or major changes, must be a deliberate decision done on the administrative account; not something just done half handedly on your normal account.
To tie in with this; when you create your admin account or have an account with differing privileges, it is recommended you create the account with the syntax: youremail+accountprivileges@… For example an admin account would look like: youremail+admin@… This ensures a clear distinction between which account is which but it also allows your to receive mail for both account at a single email address.
The most common use for the start/stop instance role is for automated start up or shut down. You are able to add an automated user to the project that will only be able to perform start and stop commands on the system. This role is recommended over say the member role for security reasons. If the automated user’s account information was ever compromised or the automation changed in some way to try and make changes to the project outside start/stop commands, they would all fail.
The auth only role is a restrictive role and has a number of use cases. The most common would be when adding a new user to a sensitive project and requiring them to change their password and setting up MFA before giving them a more powerful role. The second would be when there is a need to create users with restricted object storage access. For more information on this please see Managing access.