Edwards isn't going to win the election. But he may bring enough democrats out to vote to get Mary Landrieu re-elected.
They are running for different offices. The idea is that Edwards will help get the democratic vote out to the polls in a mid-term election that is expecting a low turnout. If it works, then Landrieu will owe him a favor. That is how the Silver Fox works. He peddles influence.
EE never ran a race to lose...his first object is winning and rubbing his detractors' noses in it. That he brings more Ds to the booth is a side benefit on which you are correct he will use as well.