Montag has gone through several mental changes from the time he first became aware of things wrong in society. Montag lacks the education and critical thinking skills needed to fully understand what is wrong; he only feels the wrongness, but he cannot think it through. Faber acts both as exposition of history (much as Beatty served this role earlier) and as a way for Montag to calm his churning mind and come to terms with his emotions.
"The whole culture's shot through... Remember, the firemen are rarely necessary. The public itself stopped reading of its own accord. You firemen provide a circus now and then at which buildings are set off and crowds gather for the pretty blaze, but it's a small sideshow indeed, and hardly necessary to keep things in line. So few want to be rebels any more."
(Bradbury, Fahrenheit 451, Google Books
Faber gives Montag the framework he needs to finally understand what has happened; society no longer wishes to be independent individuals, but instead is happy to be told what to think by the government. They can't think about things much as he cannot, but they are content with their lives, and (as Mildred) would fight against Montag if he attempted to change things. After speaking with Faber, Montag finally has the base to develop his understanding based on fact rather than on emotion.