By providing habit for creatures that may not do as well with the effects of our changes to their ecosystem, we have allow them a fighting chance. If we didn't; only the species able to quickly adapt to the drastic changes we cause would have good chances. Buffalo, for example, don't have the roaming space without preserves, and if they went extinct then the diversity of their natural roaming grounds would decrease. Less animals to graze wild grasses. But! since we have habitat set aside, they still can participate in their ecosystems.
Ecosystems need a specific type of diversity. Not just new genes, but animals and plants to fulfill specific roles, which is why just dumping random species in there is not a good idea. From a sustainability standpoint we want to keep species around, because we don't know how important their role is until it's too late. This is the reason for preserves and sanctuaries (The exception is pandas, which we preserve only for aesthetic reasons, if I recall correctly)