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.
It would take ages for me to get enough credit for a free game then. Maybe I need to buy more games lol.