Regional pilots act as wage arbitrage, this is their sole purpose. This is the same as flooding a sector with workers from China. It lowers the ultimate cost of the pilots on both sides of the aisle, by not only getting cheap labor for a small segment of planes, but more importantly, flooding the market place with cheap qualified pilots that lowers the market value of said skills of the mainline section. The majority of the savings from a regional comes from the loss of pilot pay at the top of the mainline list.

If the regionals didn't exist, management would be forced to pay top dollar if it wanted qualified pilots to join its ranks. Today this is not true, it simply has to offer better than the less than minimum wage being offered to the regional pilot, and that's why Delta will get tens of thousands of resumes while paying only half of what it did more than a decade ago.