What is the main reason that governments create natural areas called reserves?
Reserves are created to limit the use of the land or the activities that can occur in a certain area or to place a designation on the area.
Making a set area of land a "reserve" means different things in different countries. Making an area a reserve may put some sort of limit on the area or may designate the area for certain use or of certain value. A reserve may also indicate which governing body manages the area. For example, reserves may be managed by one branch of government but parks or conservation areas may be managed by different branches.
Creating a reserve may mean the land can only be used limited recreational activities such as hiking on trails and swimming in specified areas. Whereas a reserve in another country may mean the land can be used for hiking, hunting with permits, fishing with permits, and so forth.
A reserve for a certain indigenous population means that only indigenous peoples and those who have permission can enter that area.
In Kenya, the difference between a national park and a national reserve is in terms of management. The former is managed by the Kenyan Wildlife Services, which is federal, and the latter is managed at a local, tribal or council level.
In Australia, there exist two types of reserves: Karst conservation reserves which are caves of natural, archeological, natural, and historic value and natural reserves which are areas with great conservation value and that are largely undisturbed.
To conclude, the exact purpose of a reserve and the meaning of the word differs depending on the country in question.