For the two more general questions: Why are there so many color spaces? - Color spaces are 3d spaces described relatively to a reference space that encompasses all human color vision. These are usually either XYZ or LAB. Because all of the mixing and rendering is maths, each tiny variation in size of a given color space…
1. technically, pure black and white don't need to go through that node because they are the same in all variations of a certain space. However, the moment you go beyond pure black and white, the gamma correction node is necessary. For example, in sRGB the 50% gray is not equal to Linear 50% grey, rather it is equal to…