Here's why:
* Transitive verbs require a direct object to complete their meaning. The direct object is the person, place, or thing that receives the action of the verb.
* Pronouns can function as substitutes for nouns.
Examples:
* I saw her. (Pronoun "her" is the object of the verb "saw")
* They gave it to him. (Pronoun "it" is the object of "gave", and "him" is the indirect object)
* We helped them. (Pronoun "them" is the object of "helped")
Note: The pronoun must be in the correct case (nominative, objective, possessive) to function as the object of a verb. For example, in "I saw her," the object pronoun "her" is used, not the nominative pronoun "she".