The purpose of a backup is to recover data in the event of a catastrophic failure or loss of data. Copying files from one place to another doesn't address the probability of failure or recovery. Without probability, you have no idea how likely failure is, or how likely recovery of data might be. In addition, recovery needs to be tested to ensure it will work when needed.

You could copy the files onto a RAM disk on another computer and call that a backup, but you'd probably be nervous about the computer shutting off and losing your backup. Copying files to a remote disk may have the exact same probability of failure. You don't know until you investigate and calculate the probability. And if you haven't tested the recovery recently (or ever), you don't know the probability of recovery.

The probability of failure or recovery has to consider all eventualities. For example, it's now common practice for cybercriminals to attempt to erase backups of data in a ransomware attack. But it's also possible for people to accidentally delete backups. A proper backup should defend against these eventualities in order to increase the probability of a successful restore.