What answer does Montag give to Mildred's question, "Why should I read? What for?"
At the beginning of the book, Montag comes home to find that his wife, Mildred, had swallowed a whole bottle of sleeping pills. He has to call an emergency number to get help. Help comes in the form of some uneducated technicians with a stomach-pumping machine, not a doctor. When Montag asks why a doctor wasn't sent, the men say that they have so many cases each night that more machines were produced to accommodate the need. A doctor isn't needed to do stomach-pumping, just a machine with a long hose that is sent down the victim's throat to the stomach. A little while later, after all of the toxins and poisons are taken from the stomach, blood is also pumped out, filtered, and returned to the victim. Other than feeling hungry the next day, the patient is fine.
The problem with the situation is that Mildred lied about taking so many pills to Montag. She said there wasn't a problem. Montag tells her that a brush with death is a major problem. Not admitting it is also a problem. Montag reminds her of the "snake" that was inserted into her stomach, and the dire situation she is in, as the reason that she should read. He also goes on to tell Mildred that witnessing a woman kill herself for books the night before was traumatizing for him, to say the least. Then he mentions Clarisse and how she died mysteriously, probably hit by a crazy driver, and no one cares. All of these issues are the results of a society that doesn't care. That is precisely the reason that Mildred should read, according to Montag. Their society is hedonistic and maybe books can help them to understand why. Also, reading might help them to know how to stop their society from being so thoughtless and careless.
There are two major reasons Montag gives in response to his wife's question.
The first is the snake. He is talking here about the snake that was used to pump Mildred's stomach the other night after she took all those sleeping pills. He is saying that her life is empty -- so empty she doesn't even realize when she almost kills herself. Maybe books would give her thoughts that would fill that life.
The second is the example of the old woman (and of Clarisse). Both of them, he is saying, had more of a life than Mildred does. The old woman was willing to die for her books. If she was, they must be worth more than the things in Mildred's life. She doesn't even care enough to live, so how good can her stuff (the parlour walls) be?