I get where you're coming from, but the question is asking reasons for carrying out conservation programmes, not methods of conservation (e.g. if you transfer a vulnerable species to a zoo; the reason you're doing that is to protect the species). Maintaining environments and protecting vulnerable species will reduce the liklihood of extinction, which is the primary goal of a conservation programme. Introducing species to new environments isn't directly related to conservation; in fact, if the species you introduce ends up thriving "too much" in its new environment and outcompetes native species then you might actually end up destabilising that ecosystem.
For your second question, some species may have very narrow niches and require specific resources from their environment. Therefore, it's critical to maintain all the natural resources in an environment to allow all native species to thrive. When resources are in short supply, that's when selection pressure kicks in and extinction rates increase.