There's a nice GAME store very close to where I live. I've bought a couple of games there but turned down their membership offer. What do these points do?
Every purchase you make gives points, these points translate to store credit, so effectively you save yourself a little cash each time you use the card.
Every £1 you spend = 10 points.
1000 points = £2.50 credit. Doesn't seem much for spending £100.
However if you pre-order you get double points, so £40 for a typical PS3/360 games nets you £2. If you buy often it quickly builds up. Another benefit if you will quite often be offered special deals only available to card holders both in-store or online, or sometimes you get pre-order bonuses that people without cards do not get.
It isn't a massive deal, but the card only costs £3. I got mine years ago for free, threw it in with my Gamecube purchase.