Charcoal has become a popular ingredient for products to include when targeting excess oil or impurities. Although I could not find a single medical study to show the effectiveness of charcoal in treating scalps directly, I can find potential benefits to using products that contain it.
What is charcoal? Activated charcoal used in products and in medicine is made by heating substances rich in carbon such as wood, sawdust, coconut shells, etc. This is an interesting process because it allows the carbon to become more adsorbent. Adsorbent means that it can bind more molecules. In medicine activated charcoal has been used to treat overdoses and poisonings as it can adsorb these toxins quickly. It has been used in wound healing in addition to a number of other uses.
Charcoal applied to the skin is overall harmless and is not likely to irritate the skin. The theory behind adding it to products is that it may possibly absorb extra oil from the skin. If you have found a benefit in treating your scalp with charcoal containing topicals then it is perfectly ok to continue. It would likely be best for people that have an oily scalp. It is a milder alternative to help reduce the oiliness without excessively drying the skin or hair.