Writing a script is exactly like having to answer an essay question on an exam. You surely want some idea of where you're going and how you'll get there. You don't have to write out every word and nuance, but the more preparation you do ahead of turning on the camera the higher the quality of your video. Talking points are really useful for staying on track. Most importantly transition statements or visual buttons are well worth planning in advance. A video that moves smoothly between segments is the most enjoyable to watch.