I have heard people saying It's me. "It" is generally used for other than humans. So, is there any specific reason to use this phrase?
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It in this use is called the "dummy it" or "dummy subject" and represents "the situation which currently obtains," or something related to that situation. It doesn't have to refer to an identified thing.
Thus you can have
"It" is not only used for "things". It is also used for when you don't know something. When someone knocks on your door, you ask "Who is it?"
So your answer would be "It's me!"
The use of "it" for referring to humans is very limited, and this is one of those very few cases in which it makes sense.
Using "it" to talk about people
We use it to talk about ourselves:
• on the telephone:
Hello. It’s George.
• when people cannot see us:
[Mary knocks on door] It’s me. It’s Mary.
We use it to talk about other people:
• when we point them out for the first time:
Look. It’s Sir Paul McCartney. Who’s that? I think it’s John’s brother.
• when we cannot see them and we ask them for their name:
[telephone rings, we pick it up] Hello. Who is it? [someone knocks on door. We say:] Who is it?