Def is about 70% water. If you keep topping off at say 80% def load. Then over time enough water will evaporate off in the near consistant def load to start to cause issues especially in hot dry enviroments. It takes a long time, say weeks or even months maybe more if your tank is better sealed then mine for example but i saw it begin to happen when i tested my own def, when i didnt let it burn enough off for too long. So its just cheap insurance to burn off enough to keep the ballence more in line.