Are you sure they were swapping the same plane??? There are different types of 737s.

There could have been a problem with this plane...

or they looked and saw they needed to do maintenance on this particular aircraft and by flipping the routing between these two aircraft this plane would then be able to go through to one of its maintenance hubs for repairs.

The other issue is plane availability...not just for your route...but for other routes.

In Southwest "hubs" such as BNA, BWI, MDW, DEN, etc the also likely have an extra plane or two there just in case.

The may have known a part needed to be replaced and with their repair crew there they could do the work in 1-2 hrs or so....instead of causing a delay they just flipped planes.

With southwest, unlike other airlines, there is more cascading effects on arrival time stats if there is a small delay in one location.

A solution to this is sometimes flipping planes because they have already been prepped and loaded a different aircraft so they can have a quicker turnover than if the crew stayed on the same plane.

One time I was on a plane that got flipped and when we got to MDW and then the new gate they were ready to board the aircraft thus saving a potential 20 min delay. Us carry over poassangers were allowed to board first but they already had As in line to board.