The main purpose is to encode colour information on the signal used to transmit the picture. In at least some of these the colour (chroma) is split from the black and white signal (brightness information -- luminance) so that B&W televisions could continue to work once the colour system had been introduced.
You have listed 3 versions of PAL there, and they work as described above. I think NTSC (never twice the same colour) was similar, but I'm not sure about SECAM (Something essentially contrary to the American method).
Don't quote my expansion of those abbreviations.
edit: I also note that not content with removing the U from colour, you are now removing an O as well.