Why should I use a collar with the harness?

Offers some safety for unexpected behaviors (jumping, lunging, and sniffing) especially in high stimulus environment.
Prevents the leash from pulling back strap/girth strap (the belt) against your dog’s front legs.
Provides a way to guide your dog’s head without placing pressure on his trachea.
Helps prevent the harness from shifting.
Helps keep the chest strap angled upward. This prevents excessive shifting over a dog’s shoulders.
Prevents too much load on the chest strap.
Helps keep the leash fixed in position to give a dog consistent body cues.
Keeps the chest strap away from a dog's mouth (chewing) when walking on leash.