“It is necessary for us to work the works of him who sent me while it is day. The night will come when no one can work. As long as I am in the world I am the light of the world.”
Jesus recognised in the blind man something which necessitated His use of His ability to heal, ‘to do the will of Him Who sent Him. ‘It is necessary' (dei) for us ---'. There was a sense of urgency in these words, a stress on the divine necessity. Jesus was here stating that ‘we' (He and His disciples - but we can include ourselves) must be ready to take every opportunity to do God's work while the opportunity is here, because there will come a time when the opportunity is no longer here.
At that point in time it was ‘day'. It was a period when the light was shining, for the Light of the world was there. But there would also come times of darkness when the works of God could not be wrought because of the circumstances of life. And there have been such times when the works of God have been at a low ebb. They exist in certain countries today where other religions hold sway, and the light can only shine dimly in the darkness.
Jesus was speaking with a realisation of impending death. In view of that, He knew that He must carry out His responsibility to be the light of the world while He could. And He wanted His disciples to have the same sense of urgency when their turn came. None of us knows when our opportunity of service might be taken away by death, incapacity or circumstances. We also should therefore strive to do what we can while we can.
Note that it was Jesus Who connected what was to happen with His claim to be the Light of the world (see on John 8:12). He wanted the opening of the man's eyes to be taken as a lesson that all men are born spiritually blind and need their eyes to be opened by the Light of the world.
But Jesus was not saying that when He went total darkness would descend. That is why He included the disciples in His words. The light would go on shining through them and they too must work while it was day.