Backups protect your data against media failure.
The practical aspects of recovery from media failure depend on whether the media failure is on the database file or the transaction log file.
Media failure on the database file If your database file is not usable, but your transaction log is still usable, you can recover all committed changes to the database as long as you have a proper backup procedure in place. All information since the last backed up copy of the database file is held in backed up transaction logs, or in the online transaction log.
Media failure on the transaction log file Unless you use a mirrored transaction log, you cannot recover information entered between the last database checkpoint and a media failure on the transaction log. For this reason, it is recommended that you use a mirrored transaction log in setups such as SQL Remote consolidated databases, where loss of the transaction log can lead to loss of key information, or the breakdown of a replication system.