I believe originally RingDelay was used to delay ringing and the inbound call route process until the OBi received the CallerID.
There has been a fix so the OBi will start displaying the CallerID as soon as it's received even if RingDelay is set to zero, but we still have to set RingDelay if we what to test for a CallerID in the line port InboundCallRoute.
RingDelay=Auto would automatically start the inbound call route process as soon as the OBi received the CallerID, eliminating the need to guess how low to set RingDelay.
You would also allow RingDelay=0 if testing for CallerID in InboundCallRoute is not used.