When we arise to serve God with the whole of our being, instead of walking in obscurity, trending upward, in elevation, darkness is dispelled all over the earth. Men upon the earth, can see the light an be motivated to serve. God wants to showcase our obedience; (our light), so that men would have earthly models to admire and then continue to obey or be moved to follow.
John 3:21 King James Version (KJV)
21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.