Leda:
Horticultural Charcoal doesn't offer any nutritional value (and it's not the same as the type of charcoal that we use for barbequing, which contains chemicals that kill plants) but I believe it helps aerate the soil and also with drainage and the chunks of bark also help with soil aeration and drainage as well.
I always had a heavy hand at watering when I was younger and I killed a lot of plants. It was with trial and error and reading a couple of gardening books that got me to using horticultural charcoal mixed with my soil to help with drainage and keep the plant roots from retaining so much moisture. Nowadays I just buy the orchid mix that contains perlite and charcoal.
Now that I'm older, I'm really bad about letting my plants go a long while before watering but I guess our high humidity here in Florida helps.
Here's some information on horticultural charcoal:
http://www.paghat.com/charcoal...
