In the past, backups were performed on full partitions. Large partitions could take a long time to back up, and the system could not write to the partition during the process. With the advent of incremental and live backups, this is no longer a primary consideration. Another problem was that when a disk got corrupted, recovery usually was limited to a single partition. There are partition repair utilities now that can fix most problems (though not all), and only those files that can't be fixed need to be recovered.
Still, limiting the damage and the focus of recovery can be useful and remains a valid consideration.
