From Longman dictionary I see that the word necessary can be used as follows:
It is necessary (for somebody) to do something.
Something is necessary for (doing) something.
However, I saw the sentence
Air is necessary to life;
in a book which I think should be
Air is necessary for life.
Which form is correct and why?
Best Answer
This expresses a prerequisite. Here, you are saying that if you don't have Y, you can't have X.
This can be interpreted as "Y believes X is necessary", but this may not match what you think is necessary or what actually is necessary.
In the case of life, air is objectively necessary (for some life anyway), so you can say to or for.