why used who in english'
Motley has provided you with some examples of how "who" is used. As for why it is used, for the same reasons that any word is used in English; because it is useful in speaking with (or writing to) other English speakers.
Can you give us a sentence?
who is a pronoun = quién, quien in español
who are you?
who is that?
who is at the door?
who is calling?
the boy who came yesterday.
To whom did you write the letter?
I probably know more Spanish grammar than English, even though I'm learning Spanish, so I'll let an English grammarian explain.
Why? Because you sometimes you need to know the name of a person in particular.
Your question is not clear Ronnie. Please try again.