Photosynthesis produces glucose and oxygen. These are both required by humans for the process of respiration, which is how humans make the energy required to perform all cellular activities. Without access to the products of photosynthesis, the human race would cease to exist.
Photosynthesis is a process that occurs in plants. Plants use the energy they receive from sunlight to convert water and carbon dioxide into glucose. Plants use this glucose to create their own energy, storing excess glucose in plant tissues for later use. Oxygen is released as a waste product of this process.
Humans breathe in oxygen and eat the glucose stored in the plants. They convert these chemicals into energy, carbon dioxide and water in a process called respiration. The carbon dioxide and water are released back into the atmosphere, providing plants with the necessary ingredients for continuing the cycle.
Photosynthesis conducted by plants is also an important part of the carbon cycle. By taking carbon out of the atmosphere and converting it into glucose, plants help to preserve the delicate balance of the earth's carbon cycle. Too much carbon in the atmosphere can lead to climate changes that may have a negative impact on humanity both economically and environmentally.