Although I can think of no ways in which to answer this question in specific reference to The Iliad, I can certainly give you a good general reason. When you write a full-sentence outline, you have less work to do when you go to write your actual essay. Most everything is done for you already and in perfect form. If you do only a topic outline, there is still a lot of thinking to be done and ideas to add. When I write outlines, I often end up writing two: first the topic outline and then the full-sentence outline.
When writing an outline, it is important to do what it best for you. Some people might be able to write better with full sentence outlines, while others might do better with just brief thoughts and ideas. Each has it's own benefits. Full sentences are helpful because sometimes you can just copy the exact sentence from the outline into your paper. But brief thoughts might be better to bend and shape your ideas more.