A backup system with versioning allows the reconstruction of a given file or set of files at a particular point in time.
Say you have a file important_stuff.docx
and this file gets corrupted – by you, by accident or by ransomware
Without a versioning system, the backup will be corrupted along with the original. Obviously with many and large files, the problem is that storing multiple copies of the file starts to use up a lot of disk space.
Efficient backup systems will store either the original (O) and the changes so that the 3rd version can be created from O+1+2+3 or will store the most recent (C) and the reverse changes so that the third version of 4 can be created from C -1 .
Most ransomware will attempt to modify the file in place and then rename it. Backup software without versioning works most of the time (assuming the location is safe) by keeping the original and the corrupted version. However, some versions of ransomware do not rename the file and without versioning, the backups will also be corrupted.