It is the largest source of CO2 to the atmosphere (20 times larger than the amount produced by burning fossil fuels). Any small change in plant respiration rates (relative to photosynthesis) will have a huge impact on the amount of CO2 in the atmosphere.
by starting global warming with fossil fuels, we could be affecting the natural carbon cycle, which might have drastic consequences.
it's also the biggest reason that plants do photosynthesis at all - so they can use the sugars later for energy (which is obtained from respiration (glycolysis, krebs cycle, oxidative phosphorylation)). another major thing photosynthesis is used for is making sugars to put into cellulose, which is used for structural material.