As I said: "As to why a bigger plane at all, I'd guess WN's growth means they now have some routes with enough traffic, that moving 1,200 passengers a day with four airplanes (and 4 crews) instead of 6 aircraft (and 6 crews) makes sense."
73(7)s make up 62.5 percent of WN's fleet.
737 - max capacity, single class seating - 143
739 - max capacity, single class seating - 215 (50% capacity increase over 737)
763 - max capacity, single class seating - 305 (113% capacity increase over 737)
If more capacity is what you need, then more is better than more.
You might just as well question why any airline needs the A380. The answer is, they want to fly as many people as possible from an always-limited number of gates, or with the best expense/passenger-revenue ratio.
Interestingly, WN's busiest hub (233 departures daily) is now MDW (DAL ranks only 8th). MDW is not a large airport, and cannot grow (surrounded by city). If the market demand says WN could fly 50% more willing customers out of MDW...but they can't use more planes/departures (gates already full).....
Two 739s can replace 3 737s
Two 763s can replace 4 737s