As more or less said in the links above, it�s a technical requirement/process for printing documents and photographs.
What we perceive as colour on a surface actually is the result of ink absorbing some of the frequencies of the light that is striking it. When using Cyan and Magenta in the colour mix, you get a better range of colour output than if you would if you just used �red� and �blue�.
This is because cyan ink absorbs red light and reflects green and blue (remember green + blue light = cyan). If we want a printed surface to appear blue, we need both cyan ink (to absorb red), and magenta ink (to absorb green) so the only additive colour left to be reflected is blue.
You may think its all daft, but there are usually very well-established reasons.