I think that the answer to your question will be determined by the perspective of why you may think that there are more sperm produced than needed.
Nature has a way of being very economic with her use of resources. There is a concept in Modern Western Science called entropy, which basically states that processes in physics, or in nature, genreally tend to try and find their lowest state of energy. So the question of why males produce millions of times more sperm cells than was needed to fertilize just one egg is a really good one, because it means there is something about nature that we may yet not understand. My answer would be that there is an element of uncertainty in the natural world, and in order to increase the odds of fertilization, the male needs to expend a large amount of energy to be reproductively healthy. And that means making lots of sperm with lots of chances to beat the odds of missing that one tiny little target. sperms do not live long (only about 48 hours); but because there are so many of them, some are able to reach the egg to fertilize it.