Because frequently if there are no external symptoms of ich they don't die. They can live a long time with ich if they aren't showing visible signs. So I would add my infected fish to my DT unknowingly, it's still living just fine, so then I add my newly QT'd fish which gets it.
This can happen, It has happened to people. I really so no point in quarantining the first fish if you can dose the tank they are in.
And this very thing is why people suggest TTM or treating even when no symptoms are present with copper. If you take a whole 12 days out of your life to do TTM and treat prazipro do you know what the result is? Yeah didn't think you would.
To those reading this and wanting to know what to look for here is my suggestion:
Do tank transfer because then you are very unlikely to have a fish infected fish that is not showing symptoms yet harboring it put into your DT.
Include two doses of Prazi to remove flukes as well as other internal parasites.
If at this point you can simply wait no longer you at least have made it so you don't have some of the most common issues going in on your first fish. Do not cross contaminate equipment and be diligent on the times the transfers occur, very simple.
I would HIGHLY suggest those looking for intelligent practice to ignore this thought that if you can't see it in the DT you wouldn't have seen it in QT so that makes everything hunky-dory. There are steps that many well seasoned people on here follow to greatly reduce the headache of introducing disease into the tank and I promise you, they will share them with you. Do yourself a favor and look into tank transfer method and once you take the time to read the sticky you will understand how this benefits you to take the time to do it (I promise it is MUCH less time and effort than keeping your tank fallow after you add your second, third, forth fish for 72 days due to the ICH finally showing its horrible face!).
Sk8r is trying to help you avoid being the one that posts "HELP! I have ICH on 10 of my fish out of nowhere" seeking a cure that can be done in the DT with no negative to its use. Take the time to look through the different forums and you will begin to see why this post was originally put up.
Some things will lurk without noticeable markers so taking the time to understand not only what the physical signs are but also how you can effectively treat the issue with the least amount of stress to the fish can help to remove the "what if it is there with no symptom."