Imagine you have a precious photo of Grandma which you have digitised.If you back it up, a copy will sit wherever you have placed it 'just in case' something dreadful happens and your computer goes into meltdown. You will feel so relieved!
If you sync Grandma's picture from home, then go to the office, you can look at it from (gasp!) your office computer (as long as the boss doesn't mind), the tablet computer you brought with you, your best friend's laptop... Then, if you draw a pair of spectacles on Grandma and a moustache, that will be available to view on all your devices as well.
But whatever you do - back it up or sync it, locally or online, and I hope you do all of this - then do it frequently.