Probably one of the most common use case scenarios for file and folder synchronisation tools is to keep a backup of files at another destination on your network or to an external drive.
Back in the eighties and very early nineties it was very common for hard disk drives to fail as they were still in their infancy.
I can remember when you used to have to park the heads of a hard drive before you could move the computer just in case the act of moving the computer its self caused damage to the disks inside the hard drive.
Hard drives seem to be much more robust these days but they are still prone to data loss through damage, file corruption, spikes in power and user mishaps.
Being a developer I fully appreciate how important files can become and tend to keep multiple backups of files just in case of any loss or damage.
Some of the projects that I have worked on have ran over 3 - 4 years and to loose all of that information would be a complete disaster so keeping regular backups is an absolute must.