Read Committed is the default default so to speak. You can change a Database's Default to Snapshot Isolation. So that maybe one reason, they want to override this. Also the Transaction Isolation Level is for a session so if it was set to something else earlier in the session, this statement would have an effect - changing it to READ COMMITTED.
Not sure about the XACT_ABORT ON in Stored Procedures, we don't do this but then again we are only starting to get into the TRY/CATCH.