Symbolism, Imagery, Allegory
Think about it. Paris: the Eiffel Tower, baguettes, the most romantic city on Earth. And beautiful Woollett, Massachusetts: factories, smokestacks, and a whole lotta yawns.
In short, Paris symbolizes all the cool stuff that Strether never took the time to do when he was a young man. Woollett, by comparison, symbolizes the predictable yet safe life that Strether has always lived. He clearly feels insecure about this, since the first thing he tells Maria is that he's from Woollett, adding: "I feel it so that I certainly must look it, speak it, and, as people say there, 'act' it. It sticks out of me, and you knew surely for yourself as soon as you looked at me'" (1.1.21). When pressed for details, he says he carries with him the failure of Woollett (1.1.31), which Maria realizes is the failure to enjoy life.
Once he reaches Paris, however, Strether is suddenly struck with the full weight of all the cool things he hasn't done in his life. As he walks the streets of the ancient city, we learn that "It hung before him this morning, the vast bright Babylon, like some huge iridescent object, a jewel brilliant and hard, in which parts were not to be discriminated nor differences comfortably marked" (2.2.7).
This details about Paris blurring his vision is important, as the city also blurs his prejudices about what things are good and bad in life. Sometimes for better and sometimes for worse, Paris opens Strether's eyes to seeing the world in new, exciting ways.