Master Pages
 
Master page
 
Master Pages enables the developer to create a consistent look and feel for the web application. The Master page is like that of the inherited form in Windows.NET. As often, we either create a header and footer on each and every webpage or we create a header and footer control and then add those controls to each and every .aspx page.
 
A commercial web site will have a standardized look across all pages. For example, one popular layout type places a navigation menu on the left side of the page, a copyright on the bottom, and content in the middle. It can be difficult to maintain a standard look if you must always put the common pieces in place with every web form you build.
 
So finally, it is nothing but when you want to get the common look and feel of the particular part of your web we can go for master page.