pronouns - "It may have been her brother" Why do we use IT instead of HE?

I read the following in English Grammar in Use book (app):

Who was the man we saw with Anna yesterday?

I don't know. It may have been her brother.

Why do they use "it" instead of "he"?

Generally, when do we use "it" instead of "he" or "she"?

Update:

So, Does It use here as an empty (dummy) subject, or as pronoun of unidentified person?

Answer

This is a dummy it or anticipatory it. We often use it to refer to facts or the existence or nature of something. Every English sentence must have a subject, but sometimes when we're talking about the nature or existence of something, we use it as a placeholder subject. For example,

It is raining.
It was good to see you yesterday.
It was Janet at the door.

In that last example, saying "She was Janet at the door" is wrong, because we're really talking about a fact or a phenomenon, not the nature of a particular female person; we use it to anticipate the fact that is revealed later in the sentence.

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