Let’s consider the following scenario.
You create a file A.txt and save it.
Your backup solution, whatsoever it is, needs to know that it needs
to backup that file. Normally this is done by using either a command
to back it up or by using some kind of scheduler which executes such
a command in a cycle.
What happens if the backup tries to save the file.
It opens the original file, reads it, closes it (hopefully)
opens the backup file and tries to write what has been read before.
Now what if the pc gets shutdown in between the step of opening
the backup file and trying to write it - file gets corrupted.