What’s a choil on a knife used for?

“What is the purpose of the choil?”
Many knives have a small choil. A choil is the cut away area, or notch, between the heel of the blade edge and the ricasso of a pocket knife blade and between the edge and the guard of a fixed blade knife. For many knives, the reason for a choil simply ad to do with the ability to sharpen the full length of the blade edge. In some instances a choil is not necessary because the blade is wider than the ricasso making it possible to sharpen the edge cleanly all the way to the heel. While a choil often adds to the visual appeal of a blade, that is not the
reason for one to be used.
A blade that does not require a choil. However, some makes have chosen to exaggerate the choil for use as a line cutter or bird wing cutter like in the knife below.