Now if United, AA, or other large airlines that don't offer cancellation did this, that'd be another story.
One reason you'd let people cancel at the last minute is because you overbook flights. If you have 100 seats, book 105 people and then 4 cancel, that cancellation didn't cost anything.
If you book 100 people and turned away other passengers, then 4 people cancel at the last minute, now you lost 4 sales.
That said, perhaps I am dead wrong and overbooking is being used for darker purposes. Perhaps they are deliberately overselling flights at peak times knowing that they will push bumped customers to later less-full flights. If that is being done knowingly, that's a bait-and-switch operation that might be worth more than a couple percent.
No, there's no benefit to deliberately overbooking to fill a later flight given that bumped passengers are compensated. At best they're giving away a seat on that later flight for ~free, and at worst they're eating the cost of another paying passenger that can't book that seat anymore.
*source: a friend working in revenue mgmt at United
The maths of overbooking is that if you have 100 bookings on a 100 seater flight and 97 passengers show up then perhaps you could have sold 103 tickets and still not have to bump anyone. In this case you have increased revenue by 3% without an increase in costs. This can either manifest itself in an increased profit or, on a competitive route, lower ticket prices.
Of course, there will be variations in the number of people who show up and this can lead to more passengers arriving at the airport than there are seats on the plane. In this case, there is a cost to the airline, and perhaps in this big optimisation problem there are some airlines that see a bigger payoff in overbooking and bumping than avoiding overselling and minimising bumping.
I don't know about budget airlines, but full service you might expect somewhere in the region of 5% overselling. For budget, I would expect it to be higher as tickets are cheaper so passengers have less to lose by not showing up.
Nonetheless, even if it were a 1-2% increase in revenue, it would have a much higher impact on profitability. On a budget route, you could have $500 profit per flight, say a couple of dollars per passenger. Every extra ticket that gets sold that the airline doesn't have to pay costs for is a big deal, and part of the optimisations that allow airlines to offer lower fares.
You should probably consider switching to Greyhound in that case. The last few years have generally been good ones for commercial aviation but, historically, the industry lost money as whole since its inception.
One of the problems is that you may be willing to pay a few dollars more but most will just go with whatever is cheapest.
Margins in air travel are small, an extra seat sold on every flight is a huge difference, this will definitely affect revenues and profits, and will likely lead to slightly higher ticket prices. Until the race to the bottom returns and they start overbooking again.
April 22, 2017, page 11
[0] http://www.economist.com/blogs/economist-explains/2014/02/ec...
[1] Economist, April 22 print edition, page 11
Do they actually give cash?
The last time I was bumped it was ~$600 airline voucher, transportation to/from a ski resort (a perk of being bumped in the mountains!), a room for the night and a good meal voucher to cover dinner and breakfast the next day. Plus they still flew me out the next day and took care of rebooking the connection on another airline. I don't know exactly how much it cost them, but I made out like a bandit.