The quality of being useful, to which extent something is useful(noun)
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Use "usefulness" in a sentence
"Well enjoy the perks while you can because Palin will throw you under the tractor trailer wheels as soon as your usefulness is all used up."
""The U.S. will have to find a balance between their immediate needs and the long-term usefulness of their actions," says Mr. Haqqani."
"EnterRustic Fruit Desserts, a collaboration between James Beard winner Cory Schreiber and Portland baker Julie Richardson, a book whose usefulness is clear straight from the introduction, where the authors describe each pastry-fruit iteration."