Why exactly do airlines charge change fees on tickets?

We purchased a ticket for a friend to travel with us, and now we're deciding that we want to extend her stay a few days and it turns out that it's cheaper to throw away her ticket than to trade it in on the new ticket. Yes, Frontier Airlines demonstrates their so-called business acumen by having low priced tickets, but a $100 / ticket change fee. And the ticket we purchased was only $92, so...

I completely understand the logic of having a change fee but I long for Southwest Airlines, where they have no change fee for tickets except in rare circumstances. Their logic, I'm sure, is that I've already bought a seat on one of their planes, and it's in their best interest to meet my travel needs, helping establish a solid long-term relationship.

But Frontier's $100 change fee just stinks.