Why write an outline?

Why wouldn't you? You're planning a journey and this will be your road map. There will be surprise diversions and occasional roadblocks along the way but the outline will steer you in the correct direction. Be aware however no outline is perfect. Consider it a 'working document' that can change and evolve as you go along.

Importantly, having an outline will help your supervisor understand your thoughts and ideas - which leads to far better advice and feedback for you.