For various reasons, including mobility issues, some people prefer using their keyboard rather than the mouse.
Many websites list the most commonly used keyboard shortcuts, including:
Google have a number of keyboard shortcuts to make it easier to use Mail, Calendar, Docs etc.
In your documents list, for example, you can use the up and down arrow keys to move through the files in the list, and you can open the selected document by hitting Enter.
For more information, see Google's help pages:
Microsoft Office includes Key Tips. This is a feature for users who prefer to use the keyboard rather than the mouse. Every command has a keyboard equivalent, and there is no need to remember them as they are displayed on-screen.
Simply press the ALT key once to trigger Key Tips.