I imagine the barn contains family and friends lost to the walkers who Hershel believes can be brought back once a cure is found. He probably doesn't believe they are dead, but more like victims from "28 Days Later."
This is a good idea.
My first thought was that it explains the emptiness of the farm, by providing enough of an overall zombie-density on his property. This might hold if zombies tend not to congregate in groups that are too large, relative to some other distribution of resources.
The balance between humans and zombies may have been upset by the arrival of our heroes. Perhaps unless more zombies are put in the barn?
Just some musings there. I know they can be shot down.
I guess those beat my idea that he uses zombies to pull his plow.