The one fundamental argument for nature conservation is that nature is our life support system - we depend on it for so many things: food, shelter, oxygen, water, medicines, materials and inspiration to name a few. That is our starting point in explaining a species approach to nature conservation.
There are numerous arguments for protecting species including those that emphasise the economic contributions of certain species for example.
SOS promotes a species approach to nature conservation because species are nature's building blocks. By focusing on species conservation, good projects with lasting impacts often must address and resolve broader issues including habitat conservation while incorporating the needs of communities who depend on those species for survival.
So a species approach can have multiple knock-on benefits such as generating alternative livelihood solutions, enhancing protection for a specific area, reducing wildlife crime and unsustainable demand, reversing habitat degradation and even inspiring a new generation of entrepreneurs, scientists and artists.
You could say that protecting species is the ultimate nature based solution: protecting our life support system building block by building block.