側重點不同
1.It is+adj.+of側重點：有of的句子，句中的形容詞修飾的是人，作表語說明人的特質。
2.It is+adj.+for側重點：有for的句子，句中的形容詞修飾的是后面to do的部分，與人無關。
It is of first importance to spread cultural and scientific knowledge among the people.
普及文化科學是頭等重要的。
It is of convenient length.
它的長度合適。
It is for this reason that television proves such a boon to so many people
就是因為這個原因，電視才會給這么多人帶來如此大的便利。
I don't have to tell you how important it is for you to keep your mouth shut about all this
我不用告訴你，你自己也知道對所有這些事保密有多重要了吧。