What is the deal with sleeves being all purpose pockets and satchels? Was that really a thing? Did backpacks and the like not exist?

When it comes to medieval fashion, I will defer to @joannalannister.

The deal with sleeves, as I understand it, is the same deal with the medieval love of belt-pouches: namely, that they hadn’t really figured out pockets in the medieval era, so you need something to carry stuff in.