I have to admit I never use this one.
But, just suppose: You’re in Word or Excel or PowerPoint. You’ve opened up a file dialog box. It could be an Open file dialog box (by pressing Ctrl+O or using the File | Open menu, or by pressing Ctrl+F12). Or it could be a Save As file dialog box (by pressing F12, or selecting File | Save As).
And then you change your mind, and want to search the web instead. Instead of hitting Esc to cancel the file dialog box, and then pressing Ctrl+Esc to get the Start menu and then running your web browser, and then opening up a search engine, you can do all of that with one button. You could just use your mouse to hit this button, the “Search the Web” button:
Or, you could press Alt+3. When you do so, instantly the dialog box closes (with no action taken), and your web browser comes up, with a search engine displayed.
(Note to readers: If you know how to change the search engine used here from MSN Live to something else, please let me know. The standard methods don’t seem to work.)
It’s a pity this keyboard shortcut only works in Office apps and not all apps that use a file dialog box.