What is understanding of CAD backup?

backup, is another copy of the same saved drawing, it can be useful in case of damaged or unrecoverable .DWG file, we can replace it with .BAK file

main use of backup file is main recovery purpose . if we deleted the main file we can get the same through backup.

Drawing backup files are typically created every time that you manually save a .dwg file. By default the file will be saved in the same location as the .dwg and will have the same name as the drawing but with a .bak extension, such as, site_topo.bak. A backup file is an exact copy of the drawing file prior to the last save. As such, backup files are always one version older than the currently saved drawing. Only one backup file is retained at a time so newly created backups will always replace older backups of the same name.

Note: Backup files are created only if the system variable ISAVEBACK is set to1.

Backup files are essentially renamed .dwg files. You can recover data saved in .bak files by renaming the .bak extension todwg and then opening that file in AutoCAD.