Plants and trees give us oxygen that we need. 80% of plants contribute to the effective functioning of our ecosystems. If all the animals disappear the we will die soon too. It's a basic, it's the food web. One animal from the food web topple others will either die out of over population and we would have to waste money to keep balance .
We have no idea how one little thing can affect the whole picture. Nature is set up so that there is biodiversity and mutual dependence. No human society is going to thrive for long if it does not take into account protecting various species other than the humans because things can start to fall apart.
The world is abundant with ever increasing opportunities for new knowledge and development. Multitudes of information have been found via the research of animals, and even more unique information has been attained from (now) endangered species. Without the protection that can be afforded from regulation, these species will continue to decline, and eventually become extinct. We must stop this from happening.