Thing is, buying miles is normally a really poor use of your money, because the redemption rate isn’t great, and airlines devalue miles all the time. For example, the lowest option at delta is to buy 2000 miles for $70. That’s 3.5 cents per mile, but you can only expect to get a value of 1.25 cents per mile when you redeem them. Which only comes out to $25 in value, loosing you $45 — and that’s assuming you wait to spend miles for a good deal. (Redemption rate is worse during more popular flights.)
Airline miles are just not worth much, which is why people chase like hundreds of thousands of miles at a time through credit card sign up bonuses.
All rewards programs are phrased like this. You save up 10 nights at a hotel by staying there 10 times and you can spend it on a single stay.
Did you notice that the 2,500 mile flight earned/generated/conjured 2,500 miles?
(I know, I know, things can change the number you get...)