Why are menus used in user interfaces?

Answer 1

To use the UI space more efficiently... a menu allows more information to be contained in a limited space. This can mean a hierarchy of information (by grouping similar items in menus) or simply using the shifting space of active/inactive menus to optimize the screen real-estate.

Answer 2

In addition to efficient use of space. Menus may be dynamically loaded and modified during run time. Think of how you are given an option to open a previously view file in the menu. Those choices were configured by the program during the run time. Buttons may also be dynamically created at runtime but IMHO are a little cumbersome to place on the form and require alittle more overhead code wise than its worth. With Menu Items you add Items to a collection and they appear in the menu

Menus also group related functions together and clearly present a list of choices to the user. The use of sub menus let the user drill down to specific functions of a previous menu choice. To place all of the choices a menu may have on a form would quickly clutter the form with a large number of buttons.

Finally Users are much more tolerant of verbose descriptions in a menu vs buttons. You can be a little more descriptive in a menu vs a command button. A large caption in a button makes teh button bigger wasting further space. Look in MS Outlook Tools menu and you will see a menu item: Empty "Deleted Items" Folders put that on a button and see how it looks!