So what is the function of the “heel” command?

This one word command tells your dog that the animal is to walk directly at your side, not before or after you. This provides safety for your pet in public places as well as for yourself. The command keeps your pet at your side rather than running through the picnic blankets of park goers and, if you own a larger more menacing breed, makes you seem a less approachable target to would-be criminal elements.