Photosynthesis is a biological process that allows all things "green" to survive and flourish here on Earth. We as humans are able to grow and maintain our health by eating and drinking in order to gain energy - by this logic ask yourself "How do plants acquire the energy to live and grow?"
The answer is photosynthesis. The exoskeleton of the plant is effectively able to take and harness energy it receives from sunlight, and by using that energy in combination with water the plant is able to grow and reproduce. Without photosynthesis, the plant would have no way to harness the sun's energy and use it to synthesize nutrients in the soil and in the water it takes in through its roots. It would be like having all of the burgers you could ever want to eat right in front of you, but not having the strength to pick one up and take a bite.
The conversion of light into chemical energy is one of the most fascinating qualities of plants and living greenery. Without it, there would be no plants because they would be unable to acquire the energy necessary to live.
Without plants, humans would lack in a major source of nutrients - think about how much we rely on plants! Our trees provide oxygen, fruits and vegetables provide sustenance, and cotton provides fabric that can me made into the clothing we wear.