When you do a regular Save, the game demotes the existing save by that name if there is one to a backup file (and thus overwrites any existing older backup) before saving the new one. By continuing to do that, you only ever have one automatically made backup of your saved game.
When you do a Save As but don't change the name of the save from last time, it does exactly the same thing.
If you use Save As most of the time and change the name slightly each time such as by adding a number or the date on the end, then you can have 4, 6, 10, or as many as you want (until you run out of drive space) saves of the game from various points in time in case something should go wrong and you need to revert gameplay for a do-over. After a while of course you can just delete or overwrite the older saves if they are not needed. This is the best method to ensure you don't get stuck with both a game save and one auto-generated backup that are both unusable if something bad has happened and it becomes far less likely that you could lose your entire game all at once as opposed to, say, the last sim week or two of it.
