My guess is Southwest adds BWI to both IAH and ORD. Southwest can then pressure AA off of BWI-ORD, and UA off of the longer distance routes from BWI.

BWI is one airport where US once had a hub but has a diminishing role even with the AA merger, and UA has cut service even pre-Covid. It's a least of importance WAS airport relative to DCA and IAD for United, and unimportant relative to DCA for American in WAS.

Meanwhile, Southwest is massive at BWI and in general, every eastern US and central US market gets BWI coverage unless it is under 3 hours driving distance. I think Southwest will ultimately diminish United to be such a small carrier at BWI, that United only runs ORD-BWI, and American only runs CLT-BWI and DFW-BWI.

BWI is just one market but it will hold some strategic weight for Southwest into ORD and IAH