The single Menu Bar implementation on a Mac may seem illogical to new users but over time they realize that work is most often streamlined and it also assist in identifying the current or most recently used application.
 

The Menu Bar is where one interacts with an application. It's also the location where preferences and settings can be viewed and adjusted; a quick view of recent items is displayed, an application can be closed (via a Force Quit), users can log in and out, and a sleep, restart, or a shutdown can be initiated.

The menu bar changes with the current in-use application. When you switch between running applications, you'll notice that the Menu Bar changes to reflect that running application and it's associated menu items. What you'll notice about the Menu Bar is that some applications have different menu items. The Menu Bar changes to reflect the capabilities and options of the application.  Give it a try, open at least two applications and switch between them.  Observe how the Menu Bar changes.  As you can see, the first example shows the Menu Bar for Finder followed by the respective Menu Bars for Safari and Mail.

All contain "File", "Edit, and "View" but that is where the similarites end.  The Menu Bar changes to reflect the features of the application.