Development of the air cell is vital for hatching. Using an Egg Candler to read the egg is a vital tool. One can observe development of the embryo, or not. Importantly, the air cell size can be observed, allowing an assessment of the egg shell, embryo and humidity, and its affect on your egg in your incubator.
Humidity in the egg incubator affects different eggs. For example, a young chicken will most likely have thicker shells than an older chook, this means that it will loose moisture more slowly than the older chooks egg will.
This then means the Incubator cabinet humidity will need to be lower to allow the egg to loose moisture at the correct rate. So by observing the air cell grow during incubation one can get an informed view on what leel of humidity to set the incubator at.
It is best to candle the egg to test for air cell development to assess how well your eggs are doing in your incubator at that particular time of year in that location rather than simply put in a setting becuase someone said that worked for them. Your eggs and hatching of your eggs matters, so assess your eggs using the air cell development chart.