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http://en.wikipedia.org/wiki/Baseboard
It seems that this minor architectural feature exists solely to make it impossible to park furniture against walls, producing the possibility of objects falling behind shelves and dressers and desks and beds and other large, usually-immobile pieces of furniture, and also enabling the formation of cobwebs and dust-bunnies behind them.
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They also gather dust themselves, depending on the type.
That said, their purpose is to prevent damage to the wall and cover the little gap where the floor and wall meet.