At the very beginning of the movie, we see Travis Bickle's taxi emerging ominously from a cloud of steam. It has a really eerie vibe—we get the sense that we're not in a good place. After all, what good things ever move from a misty, spectral place into the light? We can only think of "Voldemort," "zombies" and "ghost ships."
…and none of those are exactly things we want to encounter in a dark, foggy, Midtown alley.
In fact, in Scorsese' vision, New York City seems to really be Hades. Travis' taxi is gliding through this dark, nightmarish world, lit with the garish neon lights that start flashing past us after we see the taxi and see Travis' eyes surveying the street. The screenwriter, Paul Schrader, said in a reddit AMA that,
"A taxi cab, for example, was a metaphor for loneliness."
The cab is a confined space, with a solitary person driving around looking for fares—a perfect way of encapsulating isolation in an urban wasteland.
Also, since this is the first shot of the movie, it's establishing location, and it's also establishing the tone or feel of the movie. Right away, we know this isn't a zany romantic comedy about a taxi driver looking for love. It's a gritty exploration of a place that isn't very nice.