Or, think of it this way. Ever sweat so much that it was just dripping off of you? Yup, me too. All that sweat that drips off of you does nothing to cool you. The base layer, if properly designed and functioning, keeps the perspiration on the skin, rather than it dripping off and being wasted, for lack of a better term. Or, you could try it for yourself. A decent base layer can be had for $10 - $15. Let us know how you make out.