......... because there are a lot of misguided souls on youtube, facebook, etc.
GC contains Praziquantel, which not only treats certain worms (flatworms/tapeworms) but also treats certain ectoparasites, such as flukes, which isn't going to help much if the med is inside the fish - hence why it is used as a bath. The med will be absorbed by the fish when added to the tank water.
Many of those same misguided souls believe that Prazi kills all worms, which it does not. For round worms one needs Levamisole, or something similar. So if one is deworming a fish Prazi alone will often not resolve the issue.