0.0039215689
is approximately equal to 1/255
.
Seeing that this is OpenGL, performance is probably important. So it's probably safe to guess that this was done for performance reasons.
Multiplying by the reciprocal is faster than repeatedly dividing by 255.
Side Note:
If you're wondering why such a micro-optimization isn't left to the compiler, it's because it is an unsafe floating-point optimization. In other words:
x / 255 != x * (1. / 255)
due to floating-point round-off errors.
So while modern compilers may be smart enough to do this optimization, they are not allowed to do it unless you explicitly tell them to via a compiler flag.
Related: Why doesn't GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)?