This is one lesson I had to learn the hard way many years ago. It can be very easy to get a fish that is carrying, or infected with, a disease in the very early stages making it very difficult (if not impossible) to visually see. I used to lose many fish to ick before I started using a quarantine tank. Many people skip this process and add new fish to their display tanks right away, using the agreement that quarantine new fish is too much work and expense. In my experience, the work and expense of dealing any type of disease in your main tank is just as costly and just as much effort as utilizing a quarantine set-up, however, it can prevent the loss of existing fish in your display tank.
The below link will provide you with some more detailed information