Tim:
primarily for COMPOSITE video, though still applicable to Y/C and maybe to YUV (but only as far as to specify scan rates). If you're using something like a USB webcam, such things have no meaning.
Anne Wilson:
The rest I didn't.
They're *mostly* to do with the colour encoding scheme. Before we had colour, the standards had different names (CCIR, EIAJ, etc.), and they specified scanning rates. Then when colour was *added*, we got a new set of standards that expanded on that.
The converse applies: They're also part of the colour decoding scheme. But, again, that doesn't really apply for webcamming.