Why is slurry of charcoal given orally to suspected poison victims?

Activated charcoal is a superb absorbant. It works a lot better than gastric lavage in most cases as it traps the toxins. It also precludes the need for emetics like ipecac (from the very old days). And with charcoal -- there's really very few downsides. If it doesn't work against the specific toxin, it still won't make things worse. It shouldn't be given to a patient with no bowel sounds, with a suspected obstruction, but that's a pretty rare combo with an acute GI poisoning.