Better unis do not accept GS in offers, but a good grade in it has been known to make the difference when they have had to decide between two people who have missed their offers.
As you can't really revise for it, a good grade will show that you have general interest and can also think on your feet. I think GS is a useful qualification as how well people do is an indication of their general knowledge, intelligence, and general skills like essay writing, maths, and comprehension (both linguistic and scientific).
Also it has been said that how well someone does in General Studies is the best indication of how well they will do in their degree, though I don't know how much evidence there is for this.
It isn't that easy, only 12-14% of candidates get an A. A range of people seem to do it too, from those who hope it will strengthen their Oxbridge application to those who are aiming for an E and an extra 20 points.