At the end of the novel, Montag is on the run from the mechanical hound, and he goes to Faber's house. It is only a matter of time before the hound, and the authorities, find them both there. They realize this, and so devise an escape plan. Montag will try to throw them off the scent by dressing in Faber's clothes, and Faber himself will take off, leaving town. Faber himself will probably be captured by the authorities for being an associate of Montag's, so he needs to get out of there two.
Faber picks St. Louis because he needs to leave town and must have a destination. Secondly, he tells Montag that he wants to "see a retired printer there." Faber wants to continue their insidious plan of planting books in firemen's houses, and he needs a printing press in order to start making those books. He also wants to start printing books just to have books around again, and to start distributing them to be read. He says of this move that he's "getting out in the open myself, at last." He has felt for years that he's been a coward by not fighting back, by doing what the government told him to do. Montag has inspired him, and so he decides to continue his rebellion in St. Louis where he has access to a printing press.
I hope that helped; good luck!