Originally Posted by HPN-HRL

IIRC (and I may not be), Southwest has had some planes fly the same route twice in the same day (a possible example: DAL-HOU-HRL-HOU-DAL-HOU... - the first and last legs are identical). In a case such as this you may not want the same flight number representing two identical flights on the same day.

With the repeal of the portion of the Wright Amendment now allowing flight numbers to continue from DAL to outside the Wright perimeter, this probably doesn't happen as much anymore.