When Southwest Airlines first launched they flew only routes inside of Texas. That way they didn’t need to ask the federal government’s permission for where they could fly (permission that was almost never granted) or how much they could charge.
They launched service based out of Dallas’ Love Field. The airport was supposed to be shut down to commercial service — incumbent airlines had all agreed to move to the new Dallas Fort-Worth — and this plan tied up Southwest’s launch in court. Ultimately Southwest prevailed, the court’s finding that the new airline hadn’t been a party to the agreement not to use Love Field so wasn’t bound by it.
With airline deregulation Southwest would no longer be limited to intra-Texas service. That meant Southwest could compete against American, Braniff and others serving the country from DFW airport from their closer-to-downtown Dallas base.