• "Why am I doing this task?" - applies to pretty much any action
• "What's the best operating system?" - in what context?
• "What is the morally right course of action here?"
• "Which of these is a better movie/record?"

Particularly useful when you spot a free-floating comparative, seems to have wider application. (e.g. you just asked it about itself.) Try it yourself, for all manner of values of "this"!