Throughout the film Stan's wife tried her hardest to draw some sort of physical and emotional reaction from her husband. This scene is no different. In this moodily lit long take, while the couple slow dances Stan's wife pulls at him trying to get him to show some emotional and physical response; she's basically proposing a more intimate moment. Stan, unable to get himself there emotionally, leaves his wife standing there. She has a bit of an emotional break while leaning against the window while she fingers baby booties.
The very fact that this entire scene is done with only one cut lets the Stan's rejection unfold over time. We have to see her do all the work in this dance. We have to sit uncomfortably as she tries to get her husband to just show her some love. Then when Stan rejects her the weight of the rejection is heavy, almost like we the viewer just got rejected.
The fact that it's darkly lit, means that we have to invest just a bit more to see what's going on in the frame. It's also a slight metaphor for their relationship. We see just the outlines of people, a lesser detailed view. A "hollower" view. Just like their relationship is hollow.
The baby booties at the end tie into Stan's wife's jealousy of their little girl. The very person she carried inside of her is the one that is "taking" her husband's attention. Stan can show more emotional interest playing with his daughter then he can a proposition from his wife. Those booties end up representing so gloomy that it's no wonder it was placed at the end of the scene.
all of the fore mentioned elements makes this scene one of the most memorable of the film. It is a perfect depiction of Stan's Wife's desperation to keep her marriage a float.