A choil does one of two jobs depending on its size. A small sharpening choil gives a stone somewhere to run out at the heel of the edge so the whole bevel can be sharpened. A large finger choil lets you put your index finger forward of the handle for close, controlled work.
Without a relief notch at the heel, the sharpened bevel has to blend into thicker unsharpened steel, and a stone reaching that junction stops dead. Over several sharpenings that leaves the last few millimeters of edge underworked and produces a visible step.
See sharpening choil.
A finger choil is big enough to take a fingertip. Choking up into it moves your hand forward of the handle and directly behind the edge, which shortens the effective blade and multiplies control for carving and detail work.
It is also safer than choking up on bare steel, because the notch stops your finger sliding onto the edge.
Every millimeter of finger choil is a millimeter not cutting. On a three-inch folder that is a real loss. The notch can also hook cardboard or rope as you draw the blade through, which is the main practical complaint.
If most of your cutting is long draws, look for a knife without one.