The "right click menu" keyboard button is surprisingly useful if you ever happen to be in a situation where you need to deal with a GUI app without a mouse.

Although these days, chances are good that some things will not be accessible at all, because many devs these days just don't care to do even the basics like submitting dialogs on Enter properly. Historically, though, Windows at least was very thorough in making sure that everything GUI could be operated entirely from keyboard. Much of that is still there, just increasingly obscure - e.g. few people know that Alt+Space will open the same menu that you can get by clicking the window icon in the title bar (the one with Maximize, Minimize, Move, Size etc). Even fewer know that when you pick Move or Size from that menu, you can use the arrow keys to move and resize the window.