With the latest release of iTunes, Apple added some very nice features. Several of them depend upon you having album art in order to make effective use of them. So it's not that surprising that Apple now offers album art through the iTunes music store. It is a bit surprising that they offer free album art even for albums you didn't buy through the music store. While this is mostly to enable the cool new album views in iTunes, there are at least a couple of other reasons.
We can figure out one of the reasons by noticing the wording on the Apple description of iTunes: "As long as you have an iTunes Store account, iTunes will automatically fetch available album art for any CD you imported to iTunes." So, Apple wants to make sure people keep their accounts with the store. This makes sense, as the number of competitors to iTMS increases, Apple will need to do everything it can to keep people buying from them.
A second, less tangible reason is aesthetic: album artwork seems likely to push more people toward iPods with color screens that can display the artwork well. I'm betting Apple hopes this will drive buyers toward the upper end of the product line.
Regardless, I'm enjoying looking at the artwork.