What they do is remove certain dissolved organic compounds as well as smaller particles, thus removing them completely from the water before they can break down (i.e., no byproducts to pollute the water). Along with that, they will often take other complexed compounds, such as copper, iodine, etc., which in most cases, is not a bad thing since they are already present in elevated amounts. The more refractory organics (terpenoids, etc.) that can be removed, the better, which is why I recommend activated carbon and good protein skimming.