One of the criterias we look at in our detection dogs is their "fight drive" , we arent meaning aggression but rather how strong is their desire for an object and, most important, how willing are they to engage us in a game with that object. While not every dog is cut out to be a detection dog, the selection process weeds out most, a lesson can be learned whether you are training a pet, a sport dog or a working dog. If we look at the dogs they will tell us how important that engagement and interaction is, otherwise the toy is meaningless.
Here are a few pics of a dog being trained to detect human remains. It is an excellent example of how the toy is only important when they know that a fight/game is involved. And not coincidently we have conditioned them to believe that the fight/game only happens right at the source of odor! This allows them to manipulate us and to get what they REALLY want.
He will even take the toy TO the odor box to make that game happen.