There are times when you accidentally delete or incorrectly overwrite an important file. Without a backup, you have little to no options to restore it.
A virus or malware has damaged or encrypted your organization's data. In this case, you want to get back to a working environment. Besides, this is also how you avoid paying a cybercriminal ransom.
Legislation and policies
You want to comply with legal or regulatory requirements for data retention. This requires a proper backup. You can read more about that below.
Suppose an employee leaves. Then it's handy to have a backup so you can fall back on this without losing that data.