This very question came up on a doggie forum that I am in:
Thus cut and pasted:
There is more :) but I felt this was more than enough...
Aren’t dog coats just a fashion trend?
Dog clothing is becoming more popular, but not only for fashion reasons. We now have a better understanding of the negative impact of the cold on illnesses or potential illnesses. Being cold can make dogs sick or sicker. Hypothermia, a condition in which the body loses heat faster than it can produce it, is a serious medical emergency.
But in the old days dogs didn't need coats!
In the past, many dogs lived in outside kennels. They built up a protective coat in preparation for winter, which warmed them like a thick fur jacket. Today many of our dogs live in the house, where they spend at least 20 hours of the day at an average of 21 degrees Celsius. This can mean they don’t build up such a thick winter coat. Although the coat changes, the undercoat is not as fully formed. We have seen this most clearly in dogs that have been brought to us, for example, in the middle of winter from eastern Europe. Their coats are often very dense and protect them like a thick woolen sweater.
How cold should it be for a dog to need a coat?
There is no exact number – the cold isn’t the only factor, but also the wind. The wind chill effect is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of air. It means that the body will feel colder, even if the temperature is the same as it was on a previous, less windy day. A general rule to go by is that a dog coat should be used as soon as people put on their winter jackets, and at the latest when also wearing scarves and gloves.
and this https://www.pdte.eu/post/2017/12/07/should-your-dog-wear-a-coat-in-winter is where I cut and pasted from.