The question is, why are these points better than cash? Quite simply, Amazon and various online trading platforms offer these points cards, which vary greatly in price. With any luck, you're paying for 4,200 points (value: 50) about 40. It thus saves a lot. However, If you buy the items directly through Xbox Live (by credit card), you pay the full price.
Conclusion: If you want to save money, buys his points online from trusted merchants and receives more goods for less money. Clear win-win situation :)