Duplicating a Slide Master is different from adding a new Slide Master. And it’s a time saving option because you don’t have to make the same changes all over again — let’s explain this with a scenario. Imagine you have formatted your existing Slide Master by applying a Background Style, adding a logo, or even adding your own Picture placeholder layout. And now you want a new Slide Master that’s almost the same as your existing one — but you want a different Theme Colors set to be used. For such a small change, it is advisable that you duplicate your existing Slide Master and then make any more changes instead of starting all over again with a new Slide Master.