Here's why:
* Metaphor: A figure of speech that directly compares two unlike things without using "like" or "as."
* In this case: The comparison is between a large crowd of people and a vast, undifferentiated sea.
The phrase "sea of faces" evokes a sense of:
* Immensity: The crowd is so large that it seems endless.
* Anonymity: Individual faces are lost within the mass.
* Uncontrollable movement: The crowd is likened to a natural force, like the ocean, that can surge and ebb.
This metaphor creates a powerful image and adds depth to the writing by suggesting feelings and associations beyond the literal description.