They lead to discovery and uncoverage, as opposed to coverage. This means that you don't need to know all the information on a particular topic, but know essential information well. Less is more. Less is better. Go indepth in your study instead of trying to cover a topic that is too broad.
Essential questions engender further and deepening interest in the subject.
They are provocative, enticing, and engagingly framed. You must go beyond the information given. Answers to essential questions cannot be found. They must be invented. Supporting questions work with the essential question to provide background and guide the work on a particular unit of study.