Why? Because this constant is needed very often, sometimes even more than pi. In fact nearly every piece of code that needs
PI eventually needs
2.0 * PI.
There are many arguments on why tau makes more sense from a mathematical perspective, most can be seen on https://tauday.com/. However, counter arguments (http://www.thepimanifesto.com/) are just as good. Proving which side of the debate is right is not the point here. The point is that it would be convenient, because every second project eventually needs it to define it globally.