Photosynthesis is important for the existence of life on this earth . In the absence of photosynthesis , would not be possible on earth . This is due to the following reasons.
i) Photosynthesis by plants provides food to animals (including human being ) . So , the survival of animals including human beings depend on the food made by plants by photosynthesis . In the absence of photosynthesis , there would be no plants on the earth and hence no animals will survive.
ii) The process of photosynthesis by plants put oxygen gas into the air . It is this oxygen gas which the animals use for breathing and respiration.
So, due to this all reasons life is impossible on the earth in the absence of photosynthesis.