Outer Space
You use a roof to keep yourself dry and safe from outer space. Look outside, above your roof, and you'll see outer space stretching above you.
You don't want to float up into outer space and get all wet, do you? No? Then you better make sure you have a roof above your head.
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Why isn't it dry in outer space, you ask? Because of all the uni (う) swimming around up there. They have nasty, wet, pointy spikes and they're all over the place.
Look closely up at outer space and you'll see all of the uni floating around up there. They're really dark, so it can be hard to notice them, but I promise you, they're there.