what advantage is there to a hacker on the network in going for the back-up copy when the original is there.
a lot of people neglect to secure their backup servers. they are much more focused on securing the live stuff. Also, backup servers tend to be monitored less, so it makes them good targets. Also, once you have access to the backup server, you can have a launch pad to several other servers since many companies back up many servers to one central one for easy management.

Once you are in a backup server you have an easier job of gaining access to the others becasue the backup server has the right to just about all data on other servers its backing up.

and if the company is good, they will have very up to date backups of current files, also many backup servers are only access durring off hours so you can sneak in and hide in the flow of traffic comming from the backup jobs.