In a monochrome, life is about edges, lines, and dark (black) or light (white) masses of shapes. The interplay of these elements will make or break your composition without color to beguile.
Shadows thus become extremely important. In a color photo, most of the time a shadow is, well, merely a shadow. On the other hand, when you start to see monochromatically, a shadow is every bit as significant as the thing itself that is casting the shadow.
In landscape, during the day, shadows are mostly determined by the position of the sun, presence of clouds, and other ambient factors. At night, everything is different. As in this photo taken near San Francisco’s City Hall, street lights cast shadows in a direction—and with a relative intensity—that you wouldn’t see during the day. Since ambient light levels are low, even relatively dim lights can cast a big shadow, making for interesting compositions that wouldn’t be possible during the day.