Backups keep your files safe.
No matter how careful you are, how much money you sink into your computer, or who you contract to keep it up and running, bad things happen. Hardware is always prone to failure, malware can cause severe system degradation, or worse, ransomware can lock all your files behind an impenetrable paywall. In all those cases (particularly ransomware), having another copy of your important documents is the best way to avoid some nasty consequences.
There are a good number of software solutions that will back up everything on your machine, and if you work in an office, people in your department may have implemented something that does it for you. Even if this isn't an option for you, simply saving the very important things to an external hard drive every so often and keeping it in a safe place will mitigate most catastrophes.