Charcoal is a traditional element of orchid potting mix with the intention of keeping the mix filter impurities and anti-bacterial benefits. Charcoal is added in to orchid potting mixes for two main reasons:
Keep nutrients in the soil and that way increase soil fertility.
Allow the flow of air through the potting mix
Retain moisture and help in good drainage.
Retains (adsorbs) excess nutrients (fertilizer), and release them later when needed.
Helps to increase the soil pH.