Why use online backup?

Safe: Having your data off-site, means it is protected from local problems such as theft, fire, and other disasters, etc.
Cost-effective: You don't need to buy or maintain any expensive hardware for backup. Also, no staff or personal time is required for creating backups.
Easy: Online Backup solutions are very easy to maintain, the backup process is fully automated, just set and forget.
Reliability: Since Online Backup services are fully automated. This solves one of the biggest problems with traditional backup, which is people tend not to do it regularly. Also there is no need to worry about damaged or lost storage mediums, such as CD's, Tapes, etc.

A good backup strategy will include both an on-site, and an off-site backup.