A strong PhD dissertation outline is like a road map without it, you won't go very far. Just say you have a party you've been looking forward to and you're attempting to drive to a certain location. You have the address, but no means of getting there. You get it there in the end, but the journey was more difficult and stressful than it needed to be. In the end, nobody shows up to the party.
It's like attempting to write a research paper, or even a dissertation, without first creating an outline.