I'm trying but failing to understand the type of revenue usage. Why did the Campbell xxx company use a penny stamp to confirm payment to them. Wouldn't just a "payment received" cancel with added signature of the employee be sufficient? Does using a stamp underneath make it more tamper proof or satisfy some legal requirement?
EDIT: While attempting to learn more about revenues I saw this statement on Wiki which possibly answers my question. "Governments enforce the payment of the tax by making unstamped documents unenforcable in court."
butterfly is correct, the physical stamp is the required proof that the tax was paid, since the stamp had to be purchased from the government.
Some of us US collectors may not be familiar with postage stamps used as revenue stamps (and vice-versa), because in the US they are not interchangeable. Indeed, one specialty area of US revenue collecting is acquiring examples of illegal usage of US postage stamps on revenue documents -- especially US postage stamps that had already been demonetized (i.e., postage stamps that were not even valid for postal use anymore).
But for some British Commonwealth countries, certain postage stamps were valid for dual use (postage & revenue). Thus, you will find some of the early GB postage stamps inscribed POSTAGE & REVENUE. Likewise, you will also find GB revenue stamps used for postage (these are not listed in Scott, but will be listed in catalogs such as the Stanley Gibbons GB Concise Catalogue).
For New Zealand, after 1Apr1882, postage stamps could be used as revenue stamps (and vice-versa). In some NZ catalogs, you will see a footnote stating that revenue cancels of postage stamps are typically 5%-15% of the catalog used price. That's just a general rule. Of course there are exceptions.
The Scott Classic will actually provide a catalog price for some examples of revenue cancels of postage stamps, when the catalog value of the revenue cancel gets past ~5%-10% of the catalog value of the postally cancelled stamp. For example, a few of the TOLLUR cancels on Iceland (see Scott Iceland #122, 123, 182, 183 for examples where the catalog value of the TOLLUR cancel is actually higher than for postally used!).
Obviously, just like collectors of postal history/covers, many specialists of revenue usage of postage stamps prefer the entire piece rather than just "tied to piece". That's where the premium, if any, really kicks in.