Ease-of-use
In both Microsoft Windows and Apple Macintosh operating systems, in other similar desktop environments and in some applications, common functions are assigned keyboard shortcuts (for example Control-C or Command-C copies the current selection).
Microsoft-style menu bars are physically located in the same window as the content they are associated with. However, Bruce Tognazzini, former employee of Apple Inc. and human computer interaction professional, claims that the Mac OS's menu bars can be accessed up to five times faster due to Fitts's law: because the menu bar lies on a screen edge, it effectively has an infinite height — Mac users can just "throw" their mouse pointers toward the top of the screen with the assurance that it will never overshoot the menu bar and disappear.
This assumes that the desired menu is currently enabled, however. If another application has "focus", the menu will belong to that application instead, requiring the user to check and see which menu is active before "throwing" the mouse, and often perform an extra step of focusing the desired application before using the menu, which is completely separate from the application it controls. The effectiveness of this technique is also reduced on larger screens or with low mouse acceleration curves, especially due to the time required to travel back to a target in the window after using the menu.
Some applications, e.g. Microsoft Office 2007, Internet Explorer 7 (by default), and Google Chrome and Mozilla Firefox 4 in Windows and Linux, have effectively removed the menu bar altogether by hiding it until a key is pressed (typically the "alt" key).
Read more about this topic: Menu Bar