Originally the penile spines acted with the foreskin as a sort of umbrella, the spines being the umbrella ribs and...well, you get the idea. This provided not only protection from the rain, but also from the sun in the hot African veldt. Humans gave up the spines when then realized that you couldn't have both reproduction and protection from the weather, finding that the shade of a tree could serve the same purpose. This led to more and better thinking abilities. Chimpanzees, on the other hand, never did figure it out and that's why such as Chongo are evolutionary dead-ends, stuck (if you'll pardon the expression) in the past with no hope of progressing into the future. Thus, we humans use them as test subjects instead of the other way around.