…or anytime you have extra $$$ in your ticket bank.
Ticket bank - Let's say your trip starts in Houston but you're based in ONT. Normally you'd be commercialed out of ONT to IAH and then go to a hotel before your trip starts the next day/night. Well, if you live in Atlanta, you can get them to buy you a ticket out of ATL to IAH instead of flying out of ONT. This way you don't have to commute to ONT first and then fly to IAH - you can go directly to IAH.
Now, if the ticket from ATL to IAH costs more than the ticket from ONT to IAH, you are responsible for the difference. Your choices are: pay from your ticket bank if you have any money there (most guys do) or have it deducted from your paycheck a couple of months or so later; which gives you time to pick up a trip and make up the difference.
In a nutshell, hopefully your trip is out a city where SWA is not the dominant carrier because if it is, your ticket index might be very low (…what the company is planning on paying for your trip...) and therefore the flight out of your home town might be more expensive and you might have to make up the difference. Hope it all makes sense.