What is the Purpose of Master page in Adobe indesign?

A Master Page is a nonprinting page that you can use as the template for the rest of the pages in your document.  Master pages can contain text and graphic elements that will appear on all pages of a publication (i.e. headers, footers, page numbers, etc.) 

A master page is a special page which will not print, unless you tell InDesign to do so. It's a page where you can set a basic layout and then all of the other pages that you will add to your document and which are based on that master page will look the same.

if u want to change anything to your pages just click on one page and change its effects remaining pages 

Master page helps you to applydesign on it to all the pages, whatever you write or put on the master page gets applied to the rest of the pages. You have master page A & B you can adjust the pages according to the the master page. It's also helpful for numbering and creating a consistent design.

The common page items like header and footer, page number, common designs etc if we create these in master page, it will appear in all pages, we don't need to repeat this in every page.

A master page is a page created to add the same design on the rest of pages, wherever we want.

A master page is like a background that you can quickly apply to many pages. Master items that appear on document pages are surrounded by a dotted border. A master are automatically applied to associated pages and commonly contain repeating logos, page numbers, headers, and footers. 

Masters commonly contain repeating logos, page numbers, headers, and footers. They can also contain empty text or graphic frames that serve as placeholders on document pages. 

A master is like a background that you can quickly apply to many pages. Objects on a master appear on all pages with that master applied. Master items that appear on document pages are surrounded by a dotted border. Changes you make to a master are automatically applied to associated pages. Masters commonly contain repeating logos, page numbers, headers, and footers. They can also contain empty text or graphic frames that serve as placeholders on document pages. A master item cannot be selected on a document page unless the master item is overridden.

Masters can have multiple layers, just like pages in your document.

Its a page which is used as a template.

 A Master Page is a nonprinting page that you can use as the template for the rest of the pages in your document.