If you have two separate hard drives installed (as opposed to having two partitions on just one hard drive), one can be used for the OS and user data, whilst the other can be used for storing backups of the user's data.
Whichever hard drive fails first, the owner still has their data on the other one.
Even today I'm seeing posts every day from people who's hard drive has failed and they lost all their data because they didn't have it backed up. Help me get my data back! I just don't understand how they can be so naive to think that a hard will last forever.