Bolts From The Blue explains the pick:
Randall Cobb fits the bill as both a WR and an experienced KR/PR. He's a little on the small side for the Chargers' style, but ability to get separation in man coverage is what the Chargers really want, and Cobb can do that. He can work the middle, make plays after the catch, break the tops off defenses and can even run block with exceptional toughness. He'll do nicely.
And why would the Chargers, with WR Vincent Jackson, need to focus on receiver so early? Well, Jackson's long-term status with the Chargers is very much up in the air. He'll be around in 2011 but after that? It's a big unknown. Cobb gives them some protection in case he leaves.
For the full explanation of the pick, check out Mocking The Draft.