Assuming you mean a dog harness or a dog seat belt, there are several situations in which it may be appropriate for a dog to wear one:
In a car: Just like humans, dogs can be seriously injured in a car accident if they are not properly restrained. A dog harness or seat belt can help keep a dog safe in the event of a collision, by preventing them from being thrown around the car or ejected from it.
During walks: Some dogs are prone to pulling, which can be uncomfortable for both the dog and the person walking them. A harness can help to distribute the pressure more evenly, making it more comfortable for the dog and reducing the risk of injury.
For training purposes: Some dog harnesses are designed to help with training, by providing better control over the dog's movements or by restricting certain behaviors. For example, a no-pull harness can help discourage a dog from pulling on the leash, while a front-clip harness can make it easier to steer a dog in a particular direction.
For dogs with medical issues: If a dog has a medical condition that affects their mobility, a harness may be necessary to help them move around more easily. For example, a dog with arthritis may benefit from a support harness that takes some of the weight off their joints.
It's important to note that not all dogs will need to wear a harness or seat belt in all situations. Whether or not a dog needs to wear one will depend on factors such as their size, temperament, and behavior. It's always a good idea to consult with a veterinarian or China Pet Belts Manufacturers to determine what type of harness or seat belt, if any, would be appropriate for your dog's specific needs.

PET SEAT BELT GG1507
Model:GG1507
Name:Pet seat belt
Material:Polyester
Specification:Double head can be adjusted 41-63cm, seat belt (55-84cm)
Product weight:200g
Applicable pets:dog
Features:Traction belt
Colour:Gray, Black, Orange, Water Green, Rose, Sky Blue