Many reasons exist to restore a database, including the following:
The best reason to restore a database is to practice the backup/restore cycle and prove that the recovery plan works. You must perform regular testing of your backup and restore strategy as a fire drill. Without confidence in the recovery, there's little point in doing backups. Remember this mantra: Backups are worthless; restores are priceless.
If a database file is missing, clicking the database in Management Studio pops up a message saying that the database is unavailable. To further investigate a problem, check the SQL Server Errorlog. In Management Studio, you can view the log under Management → SQL Server Logs. SQL Server writes errors and events to an error log file in the \Log directory under the MSSQL directory. SQL Server creates a new file every time the SQL Server service starts. The six previous versions of the Errorlog file are saved in the same directory. Some errors may also be written to the Windows Application Event Log.