You would want to use an essay type question worth zero marks when you want the student to think about what they're doing, or to explain how they got to the result they did in another question.
Currently if you set an essay type question to be worth 0 marks, you still need to review it and assign the response a mark (which you have to cheat the system into by first inputting a 1) before a grade will be sent back to the LMS.
This causes many headaches with instructors and LMS admins, and students wondering why their mark isn't showing up.
It would be better if there was a conditional in the system:
If [question worth 0 marks] and [question type = essay]:
then [don't require question review]
You can do a work around by creating a mathapp with a text box, but not everyone has access to Maple to make this.