Mavericks uses Ambient Light Sensor to keep Display Awake
Many app developers including The Verge are saying that with OS X Mavericks your ambient light sensor is watching to see of you are at the computer. Before, the computer would not go to sleep if the mouse was moving or the keyboard was being used.
The ambient light sensor is used to check the lighting of the room to adjust the brightness of the display and keyboard backlight on MacBooks.
Developer Moshen Chen originally believed it was the camera, but The Verge covered the camera and left the sensor open. They concluded that it was the light sensor, not the camera.
Source: The Verge