I am wondering about the role of charcoal as an additive to potting media. I
have seen references to charcoal "sweetening" a bark mix, but what does that
mean?

While in Wisconsin, I helped my mother-in-law repot one of her orchids, and
was surprised to find a huge chunk of charcoal in the pot (this orchid had
been potted in sphagnum moss). Since the orchid had been flourishing
especially well and we didn't know if the charcoal chunk contributed to
that, we decided to leave the charcoal chunk in it and just replaced the
moss with fresh bark (since my mother in law prefers bark).

Any insight on the role of charcoal for orchids would be appreciated.

Joanna