Yeah ich isn't a bacterial infection. However, either way its floating in the water column, it's how it spreads from fish to fish. Same with bacteria, water movement causes the bacteria to be in your water column as well. A water large water change, meaning in my opinion not 80% but around 50% could cut down on the amount of free floating bacteria and ich.
water changes aren't a solution, it's merely to go with your treatment, that's why I suggest it after treating the tank. It removes.....excess bacteria (free floating), excess protozoan/parasites (free floating), and excess stressful medication (free floating).
You and me both know that when we look at "pond" water to look for protists, amoeba, protozoan, and other parasites. We can simply just sample water and throw it under a microscope. We don't have to tredge the bottom of a pond or anything, it's free floating. Water changes after a certain increment of time after the treatment is meant to cut down on the possibility of the return of whatever it is affecting the tank and also to give the fish some freshwater