The value of biodiversity can be argued from various perspectives. Foremost, in practical terms, there's its ecological service value, as we depend on it to provide us with breathable air, useable water and productive soil, for filtration of global gases and liquids, and as the resource for all of our agriculture and much of our construction — not to mention its critical role in global temperature regulation.
We utilize biodiversity in a myriad ways, not least for food, drinks and medicines, and to provide us with compounds and organisms for research.
Biodiversity can also be argued for as a genetic resource; as a link to genetic material we may one day come to need; as a species bank for future reference.