I believe I've read that baggage-handling expenses are estimated to run somewhere around $10 for the typical bag. Instead of giving that service away for free,

I believe I've read that baggage-handling expenses are estimated to run somewhere around $10 for the typical bag. Instead of giving that service away for free, they can ask for that $10 as baggage fees, reducing the price of the ticket on a price-comparison website by ($10 * avg_bags_per_pax) and attracting more business.