What does Light Sensor do in Mobile Phones?

Hi,
i have seen in many mobile phone that it has a light sensor and i was willing to know that what it actually do does it senses light or else why it added in technical specifications with such highlighted text. If anyone knows information regarding my query answer it urgently.

thankyou,

I think they are there to adjust brightness automatically as per light you see, if you are in bright area then display get brighter if you are in darker area, display will get dull.

Or you are talking about proximity sensor ?