sa_role is required for shutting down the server.
shutdown :
shutdown will wait for completion of all running processes also initiate the checkpoint for all user databases and shut down gracefully.
shutdown with nowait :
Shutdown with nowait doesn’t make any checkpoint,it will not wait completion of any process.
It will kill all process immediately and server will proceed for winding up
Caution : Please use shutdown with nowait on your risk. It can put database under risk and delay database recovery/downtime during startup,sometimes database may go offline .