New
#1
You've just gone through a complete setup of a backup operation (what to backup, where to store the backups, etc)... wouldn't if be nice if you don't have to do that each time you want to do a backup. That's the main purpose of definition files. Once you've saved it, the next time you want to run that backup, just point to the file and say GO.
And if you want to run some set'n'forget type scheduled operations... they also use those definition files.