With my Pentax K10D, which has only 11 AF points, it is possible for the bird to drop into a gap between the AF points or drop off the edge of the pattern when I track a small object, such as a distant bird. As a result, the AF system "hunts" for anything to focus on, and I can not even see the bird following it.
Thanks to a large number of control points, they can be packed tightly so that a moving target moves smoothly from one point to the next without falling into a gap.
(You could also solve the "gap" problem by making each AF point sensitive to a larger area, but this would make it harder to see what the system is focusing on.) You might think it affects the eyes of one Portrait motive focused, but actually focused on the nose.)