Well, if it is such an older project, then it doubtful that newer technologies like razor pages exist, or even existed back then.
Your screen show shows a good old plain jane standard master page.
A master page is an “over all” template of which then you can specify child pages. This is typically used to maintain the top content, and most often thus a menu bar is used.
Hence, say a page like this:
So, your screen shot shows a master page. It is as noted a page that renders first, and then the child .aspx page is also rendered. So, for a side menu bar, or most often top menu content, then a simple standard “master” page is used.
So, in regards to webform projects?
Master pages are quite much a bread-and-butter type of form used in near all such applications. As noted, you are allowed multiple master pages. And they can’t be loaded alone. In other words, you can’t navigate to some master page, but ONLY to a form created (a child form) that specified the master page to use when being created.
So, master pages are NOT valid URL’s.
To use a master page, then when creating a standard .aspx page, you can choose to create a web page with a master page, or a page without.
Hence, when adding a web page to a project, you see these two options:
If you choose to create a page with master page? Then you see a 2nd prompt appear after asking you to choose the master page.
Hence this:
Of course, if you have more then one master page, then you see multiple choice for which master page to “combine” with the .aspx page you are creating.
So, a “master” page is common, and standard fair in webform projects.
And for “complex” web sites, then often you have nesting of such menu bars, and thus you see these options for creating a master page, of which can be nested from another. Again, often done for a menu bar going across the top, but then choosing one menu item, then the child page has a menu bar on the side – and that’s the nested master page you choose when adding pages for that one given “nesting” of the site.
Hence these 2 choices when creating a master page:
I can’t say I often used “nested” master pages, but for a large site with many pages and options, then that’s an option. And it’s a common UI choice when nesting say a menu bar across the top, and then drilling down along the side.
The default when creating a new project is to have one master page, and it will be created for you. And the default (for many years now) has been to use a bootstrap formatted menu bar for that master page.
So, for site “branding”, or the often parts that don’t change from page to page such as the menu bar and things like company logo, then a master page is used.
Think of this site (stackoverflow), and how the top menu part of the page remains the same when you navigate from page to page. So, a master page solves the typical requirement for site navigation and having a consistent top part of the page render for all pages.
Another advantage is of course when you change the master page, then all child pages in the site will reflect such changes, since they are all sharing the one instance and copy of the master page.