Why slide master is good.

Using slide master is good because it is has a thing where you can apply a certain picture that you want on every slide. if you didn't know how to use slide master you would be stuck on the way when you have to copy and paste that picture onto each slide.

You also can change the background. Again you can use the slide master to apply to all slides so you don't have to keep on coping that same background every time you go onto a new slide. 

Another thing is that you can change the font and size of writing on every slide instead of changing  it every time you go on a new slide.

So really slide master doesn't waste your time, and when you don't use it it would be wasting your time.