You know how at times you feel like you don't have full control over the player you're controlling? Like he's being artificially pulled to a certain spot or locked in? Well, using "super cancel" (that's the old WE/PES term for it) ends the AI "assistance" and gives you full control over your player.
It can be very helpful in certain situations and might be even more important if you're using the more unpredictable manual passing/crossing settings. One thing to watch out for is a slight hesitation right after you do it, but that may be avoidable with good timing/skill. I have little of either, so it's hard for me to say...
As far as how it relates to jostling, well, I guess it could be used to get you out of a jostle if you wanted to bail out. For example, if your attacking player is sealed off behind the defender as the ball is in the air, you might be able to super cancel and move in front of him or sag off a bit & hope for a lucky bounce. But I use it more to break my guy out of the "locked in" state before running onto manual crosses/through balls put into space.