Its to reward the players for bothering to pick one at all and then to give them some hints about which background might be cool and appropriate for their character, or even maybe they'll see something totally different from what they thought they wanted to do and make a more interesting character out of it.
See, this is a
bad idea, I think.
Most people have a reasonable idea of what they're building by the time they pick a background. They'll have chosen a race and a class, possibly chosen a name and even mostly settled on what the character can do - the weapons he wields, etc. When you're chosing a background people may already have a rough idea of what they want. If all the backgrounds merely grant a class skill or a +2 bonus, then this step devolves into "hey, this character wants bluff - but he's a wizard, hmm, let's see" and picking whatever possibly inappropriate background.
The backgrounds as the PHB2 presents them
don't inspire any ideas, they're just a way to distribute skills a little more flexibly. Their monotony rejects the notion of actually browsing through these things for anything
but mechanical benefit. And, suddenly, if you
think you've found a background that seems fitting
roleplaying wise but whose associated skills just don't mesh, you're kinda pushed into "selecting" another background. And it's quite clear
many of these backgrounds are quite amusing, but the one or two associated skills only vaguely cover the flavor.
I think the associated language benefit works for regional backgrounds. Predefined associated skills are almost certainly nonsense.
Fortunately it's usually not a big issue since your class probably covers most of them - but that doesn't mean I like the mechanic at all.
Backgrounds should either be at least interesting enough to
browse - or they shouldn't limit selection of associated skills. Don't impose non-sensical roleplaying backgrounds on skill choices, which is what PHB2 backgrounds effectively do: instead, let people pick a background and choose an associated skill themselves - or make sure the backgrounds' benefits are varied enough to be interesting.