You create an outline so you have a draft on which you base your further writing.
An outline is a plan or a summary for a writing project or speech. Outlines are generally done in from of lists with headings and subheadings. They help the author to order his ideas and to clarify the message of his work. By creating and looking at an outline, he can see what is missing, which points need more elaboration and which need to be replaced.
An outline evolves as the actual writing takes place and it's always subject to change.
