If you are buying a coin that's already on the market as a fully attributed coin then yeah you don't want to clean it. I buy coins that are "fresh" out of the ground so they're literally caked with mud. They look like this so they require cleaning. Properly cleaning 2000 yr old coins is a months long process involving microscope and very deliberate & precise application of effort. For some reason I find it very relaxing, almost like a form of meditation. It's a hobby that I'd love to share the results and talk about with others.

Shameless plug: You can see some of my Before/After pictures over at !ancientcoincleaning@lemmy.world :-)