It's easier to get the oil to fully cover your hands and stay in place long enough to really cling to all the spicy stuff. With a few ounces, you can really cover your hands, get throughly under you finger nails, and feel out any spots you might have missed with just soap and water. The real trick is to use dish soap after. Hand soap doesn't have the same oil and grease grabbing strength. It works like you said, but because now you've basically painted your hands in oil, you can really feel how well you're getting all the oils off your hands. You're essentially making a solution of oil and spicy solute, diluting it before washing the solution away. If the dish soap only gets 99% of the stuff off my hands, I'd rather that remaining %1 be mostly oil with some dissolved spicy solute, than %1 of the just spicy stuff.