My feeling about this, FWIW, is that Arjuna accepting Krishna as his driver for the battle is the equivalent of accepting him as his guide, or guru in life, rather than going with worldly power as represented by the army. Life can be somewhat like a battle, full of trials and tribulations, not to mention moral dilemmas.
You can see it also as if one's body is like the chariot or vehicle in which our soul traverses this physical life. Giving over the reins over to God (in whatever form or name you personally best relate to) is like saying you will live your life primarily for God, not for individual self/ ego. Another way to see this is that you invite God in to your body, to use it to experience His/Her creation and interact with it.