Watch your dog and let them tell you. If they shiver while outdoors then chances are they will definitely benefit from a dog coat or even just one to keep them dry.
Remember that we have through years of breeding, created dogs that are not equipped to stand cold or wet weather like wolves out in the wild and it can be irresponsible to leave a pet dog exposed to bad weather. When the temperature drops to freezing then all dogs could benefit from the protection a dog coat can give. Dogs like people want and need to be comfortable but they can’t tell you so you need to watch them to determine if they could be cold. If they are shivering or whining it could be from cold. Remember too that their belly has less hair and therefore more exposed. They may enjoy their walks more if wearing a coat to keep them warm or dry.
Some dogs may not like the rain and chances are you won’t either if it means getting home from a walk with a wet dog. To prevent that wet-dog smell it may be worth covering them with a waterproof coat. You won’t be able to cover them completely but even covering the majority can be enough. With the option of a mock coat, you can determine the amount of coverage for your dog.
Health & Safety
As with health, safety is something all dog owners are concerned with. Safety coats are traditionally fluorescent yellow with reflective strips so car lights will shine on them making them highly visible on or near roads. It could save your dog's life. Dog coats can also be beneficial for dogs recovering from illness or injury. A dog coat should be comfortable while allowing the dog movement and the ability to relieve themselves easily.
Small to Medium Dogs
Needless to say small dogs are more susceptible to cold and wet weather, especially hairless or fine haired dogs. Although not small, Greyhounds and Whippets tend to shiver more because they have hardly any fur or body fat and therefore a thermal or thermal/waterproof coat is ideal for these dogs. Dachshunds are generally quite chunky with different degrees of fur but close to the ground so therefore they can feel the cold more on their bellies. A tummy protector can help keep bellies warm and stop hair from dragging on the ground. If your dog seems to like the cold but hates the rain a waterproof dog coat could be all that's required.
Obviously some large breeds have thick fur and thick skin and so don’t need extra warmth like smaller dogs but if they spend a lot of time outdoors or live in unheated kennels then they too could benefit from a coat on days when the temperature drops. Remember also that large dogs generally tend to develop hip and joint problems as they get older and a coat to keep out the cold can not only be a luxury to your dog but a necessity.