usefulness：
n. 有用；有效性；有益
use：
n. 使用；用途；发挥
vt. 使用，运用；利用；耗费
It is of the greatest usefulness.
它是极有用的。
A. Ideas of immediate usefulness.
立即有用的想法。
His usefulness was over.
他不再有用了。
This is also the key to their usefulness.
这也是质数非常有用的关键。
For language learning, say, the usefulness is obvious.
这对学习语言等显然是非常有用的。