The reason is that your Macs screen produces rays and even heat while on, this heat could react with the moisture you are using and cause damage to the screen. This is very important when cleaning with a moist cloth.
Using excess vinegar in the cleaning mixture.Forgetting to either turn off your laptop completely or disconnect the power cable.Wipe with Microfiber or Soft lint-free cloth.Distilled white Vinegar and Distilled water.At Least 4 Microfiber or Soft lint-free cloth.Things you need to clean your MacBook screen.What you should know before cleaning your MacBook’s Screen.