At one level, the difference between online backups and offline backupsis that online backups are, well, online; you can make them and get atthem without having to load tapes (or hard drives ),connect your external USB hard drive, or whatever. There are twoadvantages of online backups; they involve little or no physicalshuffling around of things, and they make for very rapid restores ofdata.

These advantages should not be understated. It's much easier to automateyour backups and make sure that

Unix has a slightly odd way of handling file access permissions, inthat they're only checked when a file is opened, not every time thatyou attempt to do something with the file handle. While the choice isdefensible, in many ways checking permissions on access would be moreconvenient, since it would enable things like immediately revokingaccess to files, and very few programs actually care about this.

(And these days, it would fit better into network filesystems, manyof which operate this way anyways.)