Why does the army go to the field?

You train as you fight; wars are not fought from the comfort of your living room. I am not sure how long you have been in the military, but if you have been in long enough to have been deployed to Iraq or Afghanistan, then you know that the enemy is not fighting from their living rooms, they are bringing the fight from hills, mountains, caves, valleys, and other remote areas. By going to the field, an environment for combat is being created that will give you what you need to survive in combat and to be mission ready when you have to deploy. The old adage, "the more you sweat in training, the less you will bleed in combat," rings true. The other part of going to the filed is to build unit cohesion. Knowing you can trust, in your case, the soldier on your right and on your left, is critical in combat and by training in a combat environment in the filed, you will build that trust. Hope that helps. Best of luck.

The army by design is a field organization

Have you ever deployed? If not, when you do you'll understand why. If you have deployed then you should already know the answer.