Humans poop, because:
1) After we eat, the stomach absorbs nutrients from the food.
2) The cells takes the nutrients in (so it can continue living), and eject out the waste.
3) These wastes need to leave the body, and is gathered together
4) It is excreted out.
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If we didn't poop, the waste would be stuck in our body, and can have harmful side effects, such as build up of bacterias and viruses.
hope this helps