The reason why the ears of a cat are warm to the touch is because the blood vessels are very near the surface. The blood circulates around the body. The body temperature of a domestic cat is 100 to 103°F. This is 37.7°C to 39.4°C.
This is quite a warm temperature and when you touch the earflaps of a cat you more or less feel this temperature less a certain amount because you are not in direct contact with the blood and the blood will cool temporarily as it passes through the flap.
The fact that the ear flap (pinna) is so fragile and thin and that the blood vessels run near the surface within the flap, provides an opportunity for a veterinarian and a cat owner to administer medication via the ear flap.
Certain medications placed against the ear flap are absorbed into the bloodstream. One such medication is transdermal Methimazole.
Transdermal means through or across the skin. You can see a video on this page about the application of this medication which is for feline hyperthyroidism.