Why is biodiversity so important anyway?

 Carbon sequestration, aeration of soil, decomposition of organic matter. These are all ecosystem functions that plants, animals and microorganisms play a role in. If you were to alter or change the distribution or abundance of these organisms within their ecosystems then the ecosystem functions listed above would likely be altered as well. By changing the natural function of ecosystems you inadvertently alter ecosystem services which are ecosystem functions that benefit humans. The Millennium Assessment (a United Nations sponsored assessment of the state of the world) summarizes the importance of biodiversity with a simple formula: (Levin & Naeem, 2012)

 Biodiversity → ecosystem functioning → ecosystem services → human well-being

Each step in the formula depends on the one before it to function properly. Therefore, any change within the formula has an effect on the overall outcome. So to answer the above mentioned question, if you are a fan of clean water, clean air, and food, then biodiversity is of the uppermost importance. 

Levin, S.A., & Naeem, S. (2012). The Princeton Guide to Ecology: Biodiversity, Ecosystem Functioning, and Ecosystem Services. Princeton University Press.