Why some people let dogs wearing clothes?

It can have a few health benefits depending on the situation. Smaller dogs or dogs with thinner coats may need a little bit extra in order to keep warm in the winter months if they live in a colder area. Some dogs have skin conditions and wear special clothes for external protection. Also, do shoes count as clothes? Lots of dogs wear shoes if they regularly walk on rough terrain or if they have sensitive feet.
Apart from health benefits, some dogs wear vests and such to identify themselves as a service dog.