Reserves serve as a hedge against the limitations of management. For example, one standard management policy which promotes conservation is gear modification, such as increasing the size of mesh on nets. This decision may be quite helpful, but it cannot solve all the problems. The nets still catch slow-reproducing and exploited species such as sharks. Reserves ensure there are some places where no sharks are caught.
Reserves conserve more than fish, but also habitat which is needed especially by young fish.
Reserves also serve as reference areas to identify how an unexploited ecosystem functions. It depends on levels of enforcement and how exploited the fish are already. Roberts says we don't need perfect data to start reserves because they can be adjusted as more data comes in. They should be accompanied by other appropriate management techniques in the rest of the ocean.