The above answer may well be the one you're looking for, but just in case I'll offer another one. Your question can be read on at least two levels.
Having met Faber before the text of the novel opens, Montag thinks of him when he has trouble comprehending what he reads. The two seem to understand one another at some level, as demonstrated by Faber giving Montag, a firemen, his address, and Montag keeping it and not turning Faber in. Once Montag has made a decision that there must be more to life, and books might be the answer, he calls Faber.
Faber, then, is an alternative way of life. Or rather, he is the gateway to another way of life. He can help Montag understand what he reads, and he gives Montag an ally.
That, too, in a sense, is what Faber offers Montag.