The significant abiotic by-product of photosynthesis is oxygen. The main product of the process is glucose, which the plant uses as food. The general equation for photosynthesis is:
6CO2 (carbon dioxide) + 6H2O (water) = C6H12O6 (glucose) + 6O2 (oxygen)
It can be seen that plants are important to sustain life because they reduce the earth's carbon dioxide, and replace it with oxygen.