Transitions are what make an average speech seem polished and professional. When I tell participants in my speaking classes that overlooked transitions are one of public speakers most common problems, I can see the look of collective recognition as people remember the moments when they paused and stammered because they didn't know how to get from point A to point B. I am often asked, "What is the difference between a memorable talk and an average presentation?" Other than having a clear, well stated, well thought out purpose—I would have to say transitions.
You can't just present ideas; you must lead your audience. And that's where transitions come in; they are the maps you use when you are leading a group and you need to tell them which direction to go in next. The better organized you are, the easier it will be for you to develop smooth transitions. And the leadership evidenced when you're clearly in control adds to your power as a presenter.