FOX COAT

 It would help to know what BREED your dog is...

 Is that a picture of your dog in your profile? I won't even try to guess his breed, but he looks like he has a shorter coat. Does he shake alot when you bring him outside? If he does then in my opinion I would get him a sweater or a jacket so he can happily be outside with you.

I have short coated dogs, mine are a large breed Rhodesian Ridgebacks. I use coats or sweaters on my older and younger dogs in weather  that is cold , like under 40 degrees.  If they are to be outside for longer than a potty break I think they are important. My other adults, from 2 years to 10 are different, I don't use a coat or sweater, it is just easier to hustle them outside and back in quickly.  I worry about joints on older doga and loss of body heat on the younger ones. My dogs are very fit and lean , If you have a heavier dog, a bit chubby you have more insulation against the cold.

Bonita of Bwana

You asked this question in your brrrr thread.  Depending on breeds some do have short thin coats and require a sweater to help keep warm.  But in my opinion if you think your dog is cold buy one and try it.  They aren't that expensive.