It really depends on what you are trying to accomplish and what your video focus is. I will use some scenarios from my personal experience as an example without promoting any video specifically.
When I do Let's play style for games I will not have a script. I want to show everything as I experience for the first time with the viewer. If I am running around in panic, I want that to be my real reaction, not scripted. I also feel like that makes it more natural and enjoyable in the grand scheme.
For things where I am going to be reviewing or giving my impression on something (product, beta, game, concept, cool thing, etc) that I will have some ideas scripted down so I have talking points to use. This will prevent me from rambling on and on and keep everything short and concise.
I hope this helps!