With the water changes, you remove the toxins from the things you are killing in your tank.
In addition, the directions give you guidance whereby the concentrations of meds degrade over this period of time. With my method, they stay strong and are turned off like a light switch. No bugs have any time to build up any resistance. If any are left, they are dosed at full strength again. And in the final phase if anything is even remotely left over, the final phase is the Mike tyson KO. Followed by a 40 gallon water change to kick start all of the BB and O2 multiplication and saturation. There are tons of methods and techniques out there. I work with Discus fish, and these methods work well with them. Be sure to vacuum the bottom before each WC to remove dead waste. If you are using charcoal, you will need to discontinue when dosing meds. Once the problem has cleared, you can go back to using carbon in your filters. Just be sure to change out your carbon every 3-4 weeks. Leave your BB in your ceramic rings, bioballs etc undisturbed. Your tank will rebound in no time.
Good Luck!!!
D