An egg needs to loose a certain amount of moisture during incubation. As it does so the Air cell increases. 3 days before hatch the Air Cell should take up 1/4 to 1/3 of the egg. That gives the chick a good size area to pip into and get a breath before pipping the shell, but still be a very good size to hatch.
If incubated too dry the air cell will be larger and the chick may be too small to hatch properly.
If incubated with too high a humidity the air cell will stay small. This makes it more difficult for the chick to pip into the air cell before hatching. Even high humity and the chick can be sticky and won't dry properly if it is able to hatch at all. Way to much humidity can accumilate in the air cell and fill it with fluid. So, when the chick pips into the aircell for it's first breath, it drowns.
Matt