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.
2. Press Alt-G to change the option.
3. Press Alt-156 for some strange character.