Dog Clothes are helpful for all dogs but it depending on the dog and clothes it can also affect the effectiveness of the clothing like how dog raincoats are more effective for dogs that have longer and thicker hair such as Golden Retrievers, Or how dog coats and jackets are more effective for dogs that have short-hair like Great Danes but it all depends on the dog that you have.
So If you live in the northern hemisphere where it gets cold for months and you have a Rottweiler or Doberman or any other thin-haired dog getting them coats, jackets, sweaters, hoodies would be extremely effective for them, since these types of dogs are thinned hair dogs which makes them really not too equipped to deal with cold weather for any extended period of time, unlike Husky’s and German Shepherds their thick coats allow them to be able to handle the cold for an extended period of time.
But, yes dog clothes are helpful for especially if you have a short-haired dog and live where it can get cold or if you recently shaved your dog to get one to help keep them just a little bit warmer during the colder months because not all dogs can handle or like the cold just like humans too, were some of us can’t handle when the temperature drops below 70°F degrees while others love it when it’s below 60°F degrees, so giving your dog just that extra layer of warmth will be extremely helpful for them during the colder months.
Now to answer the question “what about clothing that’s purely for a cosmetic reason” and too that all I have to say is as long as your dog doesn’t hate it then go ahead and buy your dog some cosmetic clothes as well but I would also recommend to get them some functional clothes as well.