English Grammar – Pronouns
When we wish to speak of a name several times in succession, it is clumsy and tiresome to repeat the noun. For instance, instead of saying, “The pupil will succeed in the pupil’s efforts if the pupil is ambitious,” we improve the sentence by shortening it thus, “The pupil will succeed in his efforts if he is ambitious.”
Again, if we wish to know about the ownership of a house, we evidently cannot state the owner’s name, but by a question we say, “Whose house is that?” thus placing a word instead of the name till we learn the name.
This is not to be understood as implying that pronouns were invented because nouns were tiresome, since history shows that pronouns are as old as nouns and verbs. The use of pronouns must have sprung up naturally, from a necessity for short, definite, and representative words.
A pronoun is a reference word, standing for a name, or for a person or thing, or for a group of persons or things.
Further reading: http://www.grammarbook.com/grammar/pronoun.asp