Why are there so many different pasta shapes?
One answer to this is that Americans have a reversed understanding of the relationship of pasta to sauce compared to Italians. In Italy, the sauce is called the condimento, meaning literally it is a condiment to the noodles, which are intended to be the main source of enjoyment in the dish. When you begin to understand it that way, it makes sense that you would want to have a wide variety of sizes, shapes and textures. (What Taeraresh said is also true, there are functional differences in how the different shapes hold sauces.)