When you shut down the BI services you have to enter a (weblogic) username and password. This is encrypted and stored in a file which is read when the services are re-started. From what we can tell we can enter any username and password in here and it has no apparent effect on the restart.
a) Why are you asked to enter it on shutdown, but not on startup?
b) What function is it performing, and if we enter an incorrect one what is actually happening?
weblogic username and password is required to shutdown Weblogic servers only.
When you shutdown BI Services it will shut down all BI System components (opmn managed components) and Weblogic servers.
If you give wrong username password - it will first go ahead and shutdown System components (as they do not need weblogic credentials) and then while shutting down weblogic server it will fail. (if you are using Windows shortcuts you won't get any error)