Why do things have the shape they have? You pay with round coins for a round doughnut with a hole, pass the octagonal stop sign on the road, step into the house, turn on the round light bulb, then the rectangular TV, and ask yourself: why do objects have such shapes?
For example, do you know why the shapes of pasta are so different? The thing is that because a certain kind of it fits a certain sauce. Thin pasta like capellini is more suitable for light sauces and ingredients. Thick, ribbed pasta, like penne, is good for more viscous sauces and vegetable stews. Yummy!