The server's world is an object with properties. The world encapsulates, in itself, all the data that is relevant to its' own existence. This data is kept in memory, and can be lost very easily. Also, the world by itself is prone to data corruption.
In order to facilitate a sustainable virtual world, the server connects to a database, where it stores and repeatedly verifies its' data. The database, updated only by demand, is less prone to data corruption, and is repeatedly saved into file (on the hard disk).
The drawback of such a design is that it can become a burden to the server. By reducing the amount of queries to and from the database, and optimizing those queries, the server enjoys a stable back-up of its' data, for minimum effort.