侧重点不同
1.It is+adj.+of侧重点：有of的句子，句中的`形容词修饰的是人，作表语说明人的特质。
2.It is+adj.+for侧重点：有for的句子，句中的形容词修饰的是后面to do的部分，与人无关。
It is of first importance to spread cultural and scientific knowledge among the people.
普及文化科学是头等重要的。
It is of convenient length.
它的长度合适。
It is for this reason that television proves such a boon to so many people
就是因为这个原因，电视才会给这么多人带来如此大的便利。
I don't have to tell you how important it is for you to keep your mouth shut about all this
我不用告诉你，你自己也知道对所有这些事保密有多重要了吧。