Structure helps the audience
Structure helps the speaker
Structure improves both your content and your style, and even contributes to your confidence. It does that by facilitating your flow and directing you in each step of the way towards the next step. Thus keeping you from losing your train of thought. It forces you to prioritize your points and allocate sufficient time to each of them, thus contributing to better time management.
Additionally, in the unfortunate but quite normal event of a mishap, structure confines the damage done to a single part of the speech and prevents other parts from being affected.