It acts as a layer of gunk that coats the clean, non-oxidized metal. On the extreme end of this subject, here's a story I always bring up. A friend of mine cleans his cymbals after every gig with sandpaper. It's been about 12 years since I first met him. The tonal grooves are still quite present, although they are much more smooth than a brand new A, but still much more defined than on a Brilliant.
Some drummers prefer to use only products sold by cymbal manufacturers.
If one were to talk to a respresentitive of a cymbal company, they'd probably say that the cleaner they sell has been developed to work better on their cymbals than a brass-cleaning product sold in a hardware store. I have talked to a person at Zildjian. He said that their polish is not just a product purchased by them in bulk and squirted into Zildjian tubes, but has been developed to work best with their cymbals. Roadie In A Bottle, Barkeepers Best Friend, etc. One person recommended vinegar, although another person tried this and found that it rusted his cymbal and made it un-usable.