Booked a connecting flight, standby at the airport for non stop?
SDC allows a change of routing - standby does not.
Theory being (I think) that with a routing change, you may not get on the second leg. The what happens. Of course, going from a connection to a nonstop doesn't have that risk, which is why you probably find agents willing to do it.
For an SDC, of course since it is confirmed, a routing hangs isn't an issue.