Backup and restore go hand in hand. It is crucial that you maintain some sort of ongoing, automatic backup of important files to an external location ("external" simply means "not on the same computer where the original files are located" - e.g., an external hard drive or a "cloud" data repository ).
Think of keeping such a backup as part of normal maintenance on your computer, like keeping up with updates.
You'll want to include in the backup any important data you've created: documents, videos, music, databases, and perhaps installation software for special, favorite programs you've downloaded (such installation programs can also be downloaded again, if necessary: if they have been backed up, it'll save some time should you need to restore the contents of your hard drive).
This backup may - likely will - be essential to the success of any restoration or recovery you may have to go through, following large-scale or total loss of your computer or other device. Think hard drive failure, dropped in a lake, left in a taxi, or theft.
Should you need to restore your computer after a damaging operating system crash or failure of your hard drive, there are many tools available and different levels of effort incurred, depending on the severity of the situation.