As with the recent re-signing of S Anthony Levine to the practice squad, this is probably a situation where a player the Packers like is now recovered from a preseason injury. In Campbell's case, he had knee surgery back in August and apparently he's now ready to return to practice. Both Levine and Campbell were on the practice squad in 2010 and they probably would have been welcomed back earlier if it weren't for their injuries.
Campbell's not an exceptional player, no other team was beating down his door over the past two months, and his main skill is that he's really big (6-5, 328 lbs.). You can't teach size and wing span like that. With LT Chad Clifton still out due to his hamstring injury, they probably did need another tackle at practice.