Cylindrical coordinates: why require $0≤\varphi≤\pi$?

Question

In cylindrical coordinates:

Why is there a requirement that $0≤\varphi≤\pi$?

Answer 1

If you are asking what I think you are: If the north pole has $\phi=0$, then the south pole would have $\phi=\pi$ because you just have to walk $\pi$ radians down a line of longitude to get there.