I was thinking it would be like a giant chicken... The eggs and meat eaten for protein. They're scavengers and omnivores, so the Geyornis shouldn't be hard for people to tame. They'd be bred a little smaller and less aggressive, and more partial to feed or plants, and voila, you have a giant chicken.
A bird of that size probably laid an egg each year and had a complex mating ritual. Not the best choice to be breed at all, especially if you can just shot it, have a good meal and then turn its habitat into sheep pens.