I'm interested in where you heard that the charring light is for the purpose of removing moisture.
I suspect this is just the assumption of someone who couldn't think of another explanation. Maybe it got passed around by others who were ready to jump to the same conclusion.
The purpose of charring is to char -- to create a blackened but not burned area of organic substance, which then receives a spark or flame much more readily. Thereby an ember is created over a broad area rather than haphazardly.
Maybe with moist tobacco this is more critical, whereas many pipers find that certain cuts and types of (dry) tobacco will light well right from the get-go, without a charring light.
Before mass marketed matches and lighters became available, carrying charred material was crucial to the popular flint-and-stone method of firemaking. Old-time backwoodsmen, and nowadays bushcraft enthusiasts, would have as part of their kit an almost airtight metal container in which to char cotton or other tinder material. The container would have only a large enough hole to release smoke and keep it from exploding when tossed into a fire with a stack of fabric in it. The seal kept oxygen out and prevented the contents from burning; they only blackened.
This blackened material has the special property of catching a light very easily. Even a single spark can be coaxed into a usable ember. Sailors may also be familiar with the type of lighter that uses only a flint and a length of cotton rope: the end of the rope is charred, and quickly holds an ember when struck with sparks from the flint; even in winds that would extinguish flames from other lighters, the red ember just gets stoked all the more. When extinguished, the end of the rope remains charred and ready for the next spark. That easy-lighting quality is also the same property counted on when forming black powder from the wood board of a firemaking bow drill set.
When we pre-blacken the top layer of a bowlful of tobacco, we are preparing a similarly receptive char material. Maybe some moisture will be released as a side effect, but it will exit as vapor and steam, and the smoker will have to contend with any effect on the pipe and the tobacco it's packed with... and ultimately on the flavor and bite of the smoke. So part of the skill set of each pipe enthusiast is pre-drying tobacco to the level they find workable.
