Here's why:
* Nouns are words that name people, places, things, or ideas. They are the subject of a sentence, the object of a verb, or the object of a preposition.
* Pronouns are words that replace nouns. They take the place of a noun that has already been mentioned or is understood.
Example:
* Noun: The dog barked at the cat.
* Pronoun: He barked at her.
In this example, "he" replaces "dog" and "her" replaces "cat".
While pronouns don't technically *become* nouns, they perform the same grammatical functions as nouns, making them essential elements of sentence structure.