I believe airlines pay less to list two segments in the GDS as a single flight number than they would to list them as two separate flights.
There is no advantage to taking one of these flights, as the crew and aircraft may be different for each segment. One example: AA 1247 (AUS-DFW-ONT) today. AUS-DFW arrives at D18, DFW-ONT scheduled to depart from A15 (and ended up cancelled). And you always end up with fewer FF miles.