Babies have a very tiny and small body, plus their body temperatures are very different than ours. Therefore, you would want to keep them warm. The reason for that is if the baby gets cold they may get sick. It is very easy for them to get sick since their immune system is not fully developed.
On the other hand, check out these amazing and cutest sweaters. They look like the little adult in them. It is the sweetest sweater idea ever. Personally, I would knit many of these and create a wardrobe. Because of our babies worth everything.