The do no harm approach works like this. Your fish begins to bloat. You get it to eat a shelled pea and it ceases to be constipated. If you had been feeding a low roughage diet and that was the problem, terrific. But let's imagine the fish has a tb cyst, a growth internally, another bacterial infection... then what? You could buy a "fix" remedy - a super diluted topical antibacterial. It will do nothing. If the fish rights itself with you providing clean water, the med looks brilliant. Otherwise, the fish dies, but you feel less powerless, and feel you tried to save it.
If you look to your shelf and dose the tank with an antibiotic, you probably get the same sad result, since you can't know what your probably expired antibiotic is targeting. There are people who can regularly pull this off. I have a friend who has worked all his life in the fish business who actually hits the target with some regularity. But even he is concerned about helping antibiotic resistant bacteria.
I live in a northern climate, and fish in transit can get chilled. If the ich parasite is at the store, you have ich to deal with. If I hit it immediately, it's barely a concern. Wait 24 hours for a bottle of malachite green, or crank the heat on a heat loving strain, and fish will die. Quick responses matter. So having a bottle of meds at hand is important.
Then again, ich is not a disease. It's an organism that feeds on your fish, breeding fast and feeding fast.That makes it easier to manage.If you have the tools. We can use remedies for decent success with ich or velvet. It's easy to deworm. If you catch Camallanus nematodes quickly, you save your fish. Both are organisms we can see and diagnose. Big parasites can be treated with online photos to identify them. Micro-organisms are different for us.