Clearly, Southwest has their reasons for assigning boarding priority 0-24hrs before flight on a first-come-first-serve basis, or else they would just assign priority during payment (early-bird checkin $10 notwithstanding).
1) Loyal customers can get a "one-up" on new fliers or people who don't really care when they board and where they sit.
2) People who don't change flights within 24 hrs of takeoff can also take advantage of this --the rest might be "penalized" in this model.
Sites like "checkintomyflight.com" break apart the aformentioned model.
But, this is hacker news. So, congrats on building this service.