Because the shape and size of your body determines what things you can become really good at. If you're short and stocky you're unlikely to become a good Basketball player, a jumper or a marathon runner. If you're really tall and skinny you're unlikely to become a good downhill skier just to give a few examples.
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Body composition is important since it determines ones muscle to fat ratio. If an athlete has more muscle mass/body mass, then their performance would be easier to do.