One of the ways to backup your data is to copy your entire disk to another disk. Naturally this is called disk to disk backup. Compared to backups made to tape or other media, disk based backups are faster, more easily restored and disks have become so cheap that every one can use them.
 
Depending on the storage system, backup times are noticeably and even dramatically shorter than those required for tape backups. In many companies, this technology renders the backup window a lot smaller. The outcomes being that backups are more up-to-date, since they can be run more frequently, while application and backup servers become available more quickly for other tasks and are under less strain.

To understand why you need to use online backup services, consider the reasons why you should backup your data in the first place. The answers are obvious – to protect against hard disk failures, unintended deletions and other causes of data loss. Online backup services come in picture in the other causes.