The Army Camp is a building requirement for the Rally Spot and that's all it amounts to for most people. I keep just one in my city for this requirement and reserve the rest of my outter city tiles for resource production.
Some people build several Army Camps to speed up troop training and to increase training capacity so they can train more troops at once. I don't bother because you can build the same amount of troops just in smaller batches. As you upgrade the Army Camp; both the Training Capacity and Speed buff increase.
