This is a great comment.
The practice of adding oil is an old trick to temporarily seal the rings to help compression.
It was originally used to check if worn rings were a cause of poor compression. If an cylinder builds, say 120 psi of pressure dry, and then builds 170 psi after pouring a little oil, then the rings are worn and need replacement. This test was done with the oil poured through the spark plug hole and the engine obviously not running.
Over the years, and clouded by numerous word of mouth descriptions, this practice has morphed into what you describe. However, if this is done with the engine running, the carbon deposit can freeze the rings in place. This will increase wear on the ring landing surfaces and is very bad for the engine in the long run. Carbon particles break off and act as abrasives to wear out critical parts.
There is NO role for carbon in the combustion chamber.