Perhaps
this doc's wording makes a little more sense.
This topic is also helpful.
This is a rather old sysvar dating to when hardware specs were not so great. Today's workstations don't really have a problem with slow save times. I just leave it at the default of 50. 0 means that acad will do a full save each time.
The 'wasted space' is a history list of the changes you've made. It gets cleared when you do a full save.