Sometimes It is used to fill the subject position of a sentence, even though it carries little or no meaning.

It is raining outside.
It was John who turned out the lights.

It was nice of you to call.
It would be better if you didn’t say things like that.
It doesn’t matter if you don’t bring a gift.
It isn’t necessary to tell her.

When certain adjectives (important, imperative, necessary, essential) are followed by that, the base form of the verb is used in the “that” clause.

It is important that you arrive on time.
It is imperative that you be here tomorrow.
It is essential that he learn these things.