I work in a comic book store (plugplugplug:::THE NOSTALGIA NEWSSTAND:::plugplugplug) and so when it was noted that IDW was offering an incentive cover of the first issue of the return of JBNM, I knew I had to have one. In order to get these kinds of "incentive" covers, a store has to order a certain amount of the comic in order to earn it.
Usually incentives are just black and white printings or totally different covers, but the rarer ones are those that have actual sketches by the artist of the book on a blank cover. The amount of issues a store orders to receive them can be gratuitous which is understandable as what artist wants to, in effect, draw 5,000 sketches on a book that they've already completed. So in order to get them, it can be a real draw on the profit margin to order far more than you can reasonably assume to be able to sell.
Also, with sketch covers, you never know the quality of the work you'll be getting (I don't really own any others, but of the ones I've seen, it can vary greatly). It could be some goofy doodle, could be something too small to see--you never know. I was pleased to see this when it arrived last week. Fills up the cover, inked and perhaps not even pencilled headshot of my favorite character from the series.
One of the many incentives of working in a comic book store...