What are Faber's reasons for giving Montag his address?
Montag has killed Beatty, and he is now on the run. The only place that he can run is to Faber's house. He has destroyed the Mechanical Hound, but the government is bringing in another hound from another district. It will find him; both Faber and Montag know this is true.
Faber tells Montag to,
"....head for the river if you can, follow along it, and if you can hit the old railroad lines going out into the country, follow them. .....I've heard there are still hobo camps all across the country, here and there, walking camps they call them, and if you keep walking for enough and keep an eye peeled, they say there's lots of old Harvard degrees on the tracks between here and Los Angeles." (pg 139)
Montag gives Faber the money from his account. He fears that he may be dead by the following morning, and he wants the money put to good use. Faber promises to use the money to put their plan into effect. When he tells Montag about the camps, he adds,
"You might hole up with them for a time and get in touch with me in St. Louis. I'm leaving on the 5 AM bus this morning to see a retired printer there." (pg 132)
They watch the Mechanical Hound for a while on TV, and then Montag gives Faber instructions on how to cleanse his home so as not to attract the Hound. Faber adds,
"I'll tend to it. Good luck. If we're both in good health, next week, the week after, get in touch. General Delivery, St. Louis." (pg 136)
They still plan to get together ---- if they both survive. But since Faber is moving to St. Louis, and Montag is going who knows where, a General Delivery address is given.
The quotes are from my edition of the book, but you should be able to find them somewhere close to the page given.