Egg Slitting?

I have heard of alot of people slitting thier eggs before they hatch. Big breeders even do this.....why?
When is this..."appropriate"? What are the percentages of stillbirths doing this? What are the advantages of this, besides seeing what you have produced? Are there advantages, and disadvantages?

I think the main #1 reason people do this MOST of the time is they are simply to impatient to wait for the eggs to pip themselves the only advantages I could think of would be in the case of a weak looking clutch of eggs that you desperatly want to save or simply if your the impatient type to see what the babies look like inside

I have read that it is fairly common for the breeder to slit the eggs after the first one has pipped. This makes it a little easier for the babies to get out of the eggs, thereby increasing survival rates since sometimes hatchlings will be too small/weak to get out completely themselves....