Why these implementations, rather than (for example) “libm” or “gsl”?
Functions in system libraries are usually implemented for accuracy and generality; for example, the “sine” function would likely give you the sine of every angle representable as a double-precision number, accurate to one least-significant bit. But for a particular modeling application, you may know that you are dealing only with angles within a restricted range (for example, “up to one complete turn”); and you may know that “five significant figures” will be at least as precise as other approximations in the model. In the case where “time to solution” or “cost to solution” depends on the performance of the math functions, these implentations can make the difference between a mere research tool and commercial viability.
