Dogs don't have to have blankets in the winter as long as the dog is sleeping inside the house where it's warm which they should be in really cold weather.
But having a dog blanket for the dog in their dog house is good to help keep the dog warm while they are outside in the cold.
But the blanket and dog house cannot keep the dog warm enough during really cold weather so they can still freeze to death.
In the house a dog blanket is also nice for the dog but it's not a requirement.
It's OK for dogs to be outside in the cold weather for a few hours or so but if it's really cold outside then an hour at most is enough for the dog to be outside.
If the weather is 40 F or above then the dog is OK for around 2 to 3 hours outside.
However you should always bring your dogs inside the house where it's warm because dogs can freeze to death just like humans can.
If the weather is too cold for you outside then it's also too cold for the dog to be outside for too long.
Even though the dog has fur the dog can still freeze to death especially when the outside temperature drops below 30 F degrees.
If it's below freezing then the dog can easily freeze to death even though the dog has a dog house.