t.f.h. "Aquarium Plants" book & charcoal in the substrate

The book is (c) 1987, 128 pages with fairly good descriptions of plants including
cultivation suggestions. There are many Cryptocorynes and Aponogetons!
The pictures are very good color drawings (not photographs) but show the identifying
features very well.

For the A. madagascariensis (and the other Aponogetons) it suggests a mixture
of 1/3 sand, clay and charcoal. Has anyone else heard of the use of charcoal
in substrates or have any experiences with it? What is the purpose of this
do you suppose? To provide elemental carbon or CO2? For adsorbtion?

 - Steve