Because you wouldn't want to deprive your plant of all it's nutrients. By flushing with 1/4 strength nutrients it allows a little to be available to the plant. If your doing the final flush you want the plant to use up it's stored energy so the smoke is smooth, but any other time it would be counter-productive. This is what I have read and it makes sense to me. I don't really flush in my current setup because I don't use chemical fertilizers, but I will definately use 1/4 strength flushes when I try a soiless medium.