Beatty visits Montag when he doesn’t come to work because he realizes that Montag is experiencing a crisis of faith about books.
Mildred tells Montag that he has been lying in bed five hours later than normal. He tells her he is sick, and asks her to call Captain Beatty. She tells him he has never been sick before. Mildred argues that he should have already gone to work, and he is not sick. Then Captain Beatty arrives. She refuses to tell him Montag is sick.
Beatty is completely aware of what is going on. He has seen it before. He proves Montag with a lecture, explaining why society has become the way it is.
"At least once in his career, every fireman gets an itch. What do the books say. … Well, Montag, take my word for it, I've had to read a few in my time, to know what I was about, and the books say nothing! (Part I)
Beatty explains all of this to Montag in the hopes that he will understand that there is nothing in books worth keeping. He wants Montag to not make the mistake of throwing his life away. He knows that Montag might choose books, and perhaps feels that he already did. Maybe he does not care. Either way, he knows that he has done his job to explain to Montag why he should keep his.
It doesn’t work. Montag does return to his profession as a fireman, but he does not give up books. He falls for them, and goes deeper and deeper into exploring them. He looks up Faber, and gets a second opinion. He has to choose between Beatty and Faber’s version of events, and he chooses Faber’s. He is one of the few people who decides that society needs books.