The key used on Windows for numerous reasons.
1. For simple access to menu items. Why did they make the mouse then?
2. For simple access to widgets, or window items. Once again, a complete use of no mouse.
3. To make special characters. By holding down Alt, typing a number on the number pad, and letting go of alt, you can create little e's like é, or a's like å, or so on. Unfortunately, this is a blatant copy of the Macintosh operating system BUT in a much more confusing interface. Once again, for example, pushing option-e then e on the Mac gives you é. On windows... i don't even know the combination. It's too confusing.
4. Used in the ominous key command Ctrl-Alt-Delete.
1. Press Alt-F for the File menu.
2. Press Alt-G to change the option.
3. Press Alt-156 for some strange character.