Having the keyed foreground interact with the lights of the background footage is the main thing light wrap is accomplishing. This shows a perfect example of why that is so important. The woman and the edges of the train window look much more realistic due to the way they interact with the train station lights. Both are lightened as the lights from the station move by, and are darker when passing walls with no lights.
In the still image, note the glow on the woman's head and the lightening of the window frame.
The next example uses a stationary video for the background and wraps that around the woman as she walks. This isn't as pronounced as the above examples, there are no moving lights, but it still helps make her look like she's part of the environment. Compositing is tricky and it's often the small, subtle things that make for a great greenscreen composite.
This example shows the Light Wrap plugin applied side-by-side with no light wrap. You can see how the foreground and background are blended together to create a more realistic blue/green screen composite. Even if your actor and background are mostly stationary, light wrap can be a huge benefit.