Here's why:
* He's starting to question his work: Montag is a fireman, whose job is to burn books. But he's starting to feel uneasy about this. He's been exposed to the beauty and power of literature through his encounters with Clarisse and Faber.
* The book is special: The book of poetry represents something different, something he can't bring himself to destroy. It symbolizes the beauty, knowledge, and freedom that books offer.
* He wants to keep it safe: He hides the book in the incinerator, a place where it's hidden in plain sight and unlikely to be discovered by his superiors.
This act is a pivotal moment in the story. It marks the beginning of Montag's rebellion against the oppressive society and his journey towards self-discovery through literature.