I have used it in work and personal situations to resolve seemingly impossible situations. In the work situation I have used it to resolve differences with customers. We have had situations where both parties are unwilling to budge on key points and this question can sometimes start the ball rolling for resolving the disagreements.
One a personal note, I also used it when buying our current house. We reached a situation where it looked like it was all about to fall through, after about 4 months of negotiations, solicitors and stress. The estate agent seemed to be unwilling to work towards sorting out a number of problems. This simple question highlighted a way to take responsibilty for the problem and how to solve it.
It can also be used for setting goals. If you decide what you want to achieve then ask this question it can highlight a step you need to take to reach your goal. Asking this question again will identify what is needed to achieve this step. Repeatedly applying this question can help to identify the steps needed to reach your goal.
For example, if you wanted to start a business then …
What do I need to do to make it happen? Start a business
What do I need to do to make it happen? Identify a business to start
What do I need to do to make it happen? Confirm the business will work
What do I need to do to make it happen? Learn about starting a business
What do I need to do to make it happen? Read books or take a course
Not the best example in the world but hopefully you get the idea. A seemingly daunting task can be broken down into a number of steps. It can be done several times to identify different actions that need to happen, for example in the above case you may also need to locate financing, an office location, products. etc.