As the others have said, I’ve always assumed it’s about cooling. In a marathon, even running “within yourself”, you still end up getting pretty hot even if the conditions are cool.
I don’t particularly like them however. There are a lot of drawbacks. The chafing under the arms (about 3 inches down from the armpit on the arm, and the same spot on the side of the torso) is horrible, and I have to body glide / vaseline up that area significantly. And then there’s the nipples. In any training I will ALWAYS wear a tight fitting base layer. Then you don’t have any problem. But run more than a 10k without some form of nipple protection and it’s a lot of pain for days after.
I now use these which are great. They actually stay there, unlike plasters, or vaseline which rubs away pretty quickly.