Coffee cups often come with paper sleeves that help to hold in heat. More specifically, they keep the hot liquid in the cup from burning the hands of someone holding it. Coffee might not be boiling, but no one wants to have to constantly let go of their mug because of a higher temperature. Disposable cups have paper sleeves while a leather coffee sleeve is a reusable option. Something like this goes onto a permanent cup made of plastic or stainless steel.
The two advantages of a permanent sleeve are that it does a better job and it is attractive. Obviously, it is reusable, but the soft surface also tends to be quite cool because the slow rate at which it transfers heat. Leather is comfortable to hold and provides a thicker insulator than some dinky strip of paper, no matter what the print looks like.