The tabulator key (Tab) can be used for auto-completing command names and directory paths when dealing in the terminal, or to move the keyboard input focus when dealing with normal windowed applications. Using the Tab key to switch focus, you need to switch between keyboard and mouse much less often, which normally results in improved workflow and productivity. Additionally, you can use Ctrl+Tab to switch to the next open tab (in web browsers, editors, ...) and Ctrl+Shift+Tab for the previous one. Try it sometime.