Travis Bickle: All the animals come out at night - whores, skunk pussies, buggers, queens, fairies, dopers, junkies, sick, venal. Someday a real rain will come and wash all this scum off the streets.
Travis Bickle: [Into the mirror.] You talkin' to me? You talkin' to me? You talkin' to me? Then who the hell else are you talkin' to? You talkin' to me? Well, I'm the only one here. Who the f**k do you think you're talking to? Oh, yeah? Okay! [whips out gun.] Huh?
Travis Bickle: Loneliness has followed me my whole life, everywhere. In bars, in cars, sidewalks, stores, everywhere. There's no escape. I'm God's lonely man.
Answer: It helps him create a tolerance for pain. As we see, he certainly gets a lot of that dished out to him.