RE: pp
Hi Havva
Thank you for your question.
The master slides are used to set up the formatting for the slides in your presentation, for example, formatting fonts, bullet shapes, backgrounds.
Any changes you make to the slide master affects the majority of slides in your presentation. The point of making formatting changes on the slide master is so you don't have to go to each individual slide in the presentation and make the same change (e.g. increase font size) to each slide.
I hope this helps.
Amanda