By trying to preserve species that may be directly affected by ecological changes
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)
It is an area that is protected by law from human interference.
In all countries there are areas with ecosystems of special interest, like forests, coral reefs,oasis, and any other type of ecosystem. The country might be interested in that area because of an endangered species or a rare one. Or the area could be of an international interest.
In this case and in order to prevent any deterioration caused by human activity to it, regulations are set by the government to protect this area. Some times it is fenced, some other it has its own natural boundaries. Guards are contracted to monitor the application of the laws in this area, and of course there are penalties for the people who do not follow the rules. This area is a preserve.
Habitat destruction is the leading cause of extinctions (see graphic immediately below). Preserving habitat therefore helps to preserve biodiversity.
In addition to the many roles that diverse species play in ecosystems, humans directly benefit from biodiversity in many ways as well. Some reasons that we need biodiversity are summarized in the graphic below.