A nagging child possesses a lot of power. No matter how much resolve you have, the constant 'I want that!' wears you down over time. Occasionally, that means you'll acquiesce and buy your child something that they spotted out of the corner of their eye when you were out shopping. At that moment, it's the most important thing in their whole universe, and they simply can't imagine life without it. It's just not worth the argument to say no.
Two weeks later, the fascination has passed, and whatever the must-have item was is now under a bed, or in a draw, or behind a cupboard. Two weeks beyond that they'll forget they ever had it all. It doesn't matter to them though, because you're going to the shops again this weekend, and they're bound to spot something else they just need to have. The wonderful process all begins again!