It's an enticing incentive to the potential buyer, to offset the additional $1 they're paying over the majority of DC titles.
It's a sales pitch.
Putting the ad inside the book would kinda defeat the purpose (and ads do mention this).
And really, it's still a MILE better than the Marvel ads in the 80's that had stuff like a third of the cover taken up with the 'Win A Bike At Toys R Us' etc. One of those annoys the hell out of me as it's on one of the key Dark Phoenix Saga issues, issue 137. 'THIS COMIC COULD BE WORTH $2500 TO YOU. BUGGER THE ART, THIS IS AN AD, BITCHES'.
This is far less obtrusive, and is good promotional tool for a flexible market, and value proposition for the customer (even though Marvel even admitted it doesn't need to charge $3.99 in the first place).
It could be far worse.![]()