I've got two reasons:
(1) I buy everything WotC produces for D&D. I've declared their material my "baseline" for campaigns. If other players want extraneous material, they have to buy the other source material. Helps me manage the RPG budget and also supports a company that (IMHO) has done a good job overall, contributes much to the industry/hobby, and provided me with RPG enjoyment for 20 years.
(2) Same reason I buy all the WotC plastic minis--ready-made props. I collect props of all kinds for gaming. I've got a computer folder of fantasy art for use in games that's at about 1.4 GB. I've got at least one of each mini from the Harbinger and Dragoneye set, with 8 of the "grunt" units (e.g. elven archer, dwarven fighter, skeletons), 4 of the "special troops" (e.g. hobgoblins, ghouls, elf spearmen), and 2 of the often-paired monsters (e.g. giant spider, umber hulk, gargoyle). I just bought a case and a half of the Archfiends set to start that collection.
I'll use some of the maps as player handouts, others as inspirational material (Yes, I've already downloaded all the maps from Map-a-Week, but it's easier to find something printed). If one of the maps really strikes my fancy for use in play, I'll scan-print it to a large format printer (prints 42"x84" in 32-bit color).
Obviously from the above I devote a fair amount of money to my hobby. I don't generally attend cons, so that saves me a bundle, I'm well out of school (even grad school), and luckily my wife is a gamer.

So maybe WotC is targetting people like me, folks with a disposable income dedicated to their hobby of choice. I generally agree that this particular item would be a "luxury purchase" rather than a core need, or even want.
I really have a third reason-I love nice maps-but someone's already posted that possibility.
Eric