If you like dog clothes, your other interests may include: children's beauty pageants, bestiality, and making your pet hate you.
It would make sense to make clothes for Fido if he was cold. However, seeing as most dogs have fur, and fur has the unique property of preserving heat, ninety nine percent of the time a dog won't need additional layers for a trip outside.
Despite this, there are people in the world who buy clothes for their dogs- (cats and gerbils aren't safe either, for that matter). These people are usually rich, and are showing their generosity by providing the same overpriced designer clothes they wear for their animal companions. A heartfelt act of kindness- anyone living in a third world country is more comfortable naked anyway.