Louis has an Equafleece that I put on him when he's very wet after rainy walks. He dries more quickly and my house stays a bit cleaner.
William also has an Equafleece but he wears his during walks in heavy rain, not after. He doesn't much like getting wet and as he's castrated and has a thick coat he takes ages to dry. If he's got really wet he won't settle when we get home but gets very agitated and shivery and rubs himself along the sofas. So it's easier for both of us to keep him as dry as possible in the first place.
I don't think they need coats to keep them warm as they're both very active, but that might change when they're older and less active.