Charcoal

Charcoal is one of the many drawing practices available to people within illustration, its medium allows for its user to freely draw what they see without worrying about making mistakes as the markings of charcoal allow for mistakes to be turned into improvements. It's very easy and fun to improvise with, not only that but it can help its user study Tonal Value which would then further their artistic capabilities.

Due to its ease of access and the fact that it is inexpensive this makes a good medium to use when illustrating.