Apache Guide: Apache Authentication, Part II
The second reason is ease of data retrieval. A database--at least, a decent database--provides you with a language (usually SQL) for querying the database for specific information.
The third reason is data integrity. Since a database handles a lot of things for you, which you would have to handle for yourself when using a text file, you are less likely to screw up your data, and lose information, when using a real database.
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