Slide Masters Always a Problem

Here's how I put it: Master Layouts let you add slides that already have the basic pieces you want to use.

For example, if you want  the Slide Title to appear left-justified in a blue banner across the top of all the slides, adjust the placeholder on the Master Layout(s) that way. Then you won't have to adjust the look of the title box on each individual slide.

Another example: If you want a certain photograph to appear in the background on multiple slides, you only need to put it on a Master Layout (instead of copying and pasting it to multiple slides). 

You can create as many Master Layouts as needed. However, it usually doesn't make sense to create a layout that'll you'll only use for 1 slide. Instead, start with whatever layout is closest to what you want (or start with a blank layout), and then revise the slide as desired. 

Here's a bit more info more that might be helpful:

On Master Layouts, you can insert a Placeholder. A placeholder defines an area for inserting a specified type of content on a slide. (A Content placeholder can hold anything.) 

On a slide using that layout, the placeholders will appear on the slide and in the Timeline. They can be edited, moved, reformatted, etc. 

On Master Layouts, you can also insert other objects via the INSERT tab:

On a slide using that layout, those objects will not appear on the Timeline. And they won't be editable nor moveable. Also, they will appear below/under any placeholder boxes or objects inserted directly on the slide. That's true even if you bring them to the front/top of the timeline in the Master. 

Best wishes!