Because dogs are predatory by nature, their instincts dictate that they channel their energy (and built-up stress) into hunting activities. For different breeds, this means different things, such as eye-stalking, digging, tracking, etc. However, the culmination of every hunt is the bite, because the goal of hunting is killing the prey.
So the act of biting, and specifically biting and carrying an object, represents a successful “hunt.” And then chewing and ingesting completes this cycle of emotional grounding.
Try to keep this in mind when working, walking, and training your dogs. Keep their jaws busy and you will have a happy companion!