Writing it down seems to have a similar benefit, but it usually takes longer for most people to write a speech than speak one. Moreover many people write differently than they speak, which makes writing unsuitable as a form of rehearsal.
For the most part, I do talk the same way I write. Fortunately, writing lets me take back something that I really shouldn't say out-loud, before I post it out-loud.
I don't actually write a speech, as much as I write the salient points of the speech. That is probably one of the reasons that I tend to have short speeches. Which worked for me as the introductory speaker for the group presentations in school. Basically, my parts of the presentation boiled down to, "Hi, this is who we are, this is what we're talking about, and this is who is up next." Then I'd let the people that can talk for "years" go ahead and talk, while I do the support stuff like change slides for the people that have issues with that.
I will say that the group papers were graded well. Partially, I'd like to think, because one person wrote the introduction, the conclusion, and tie the various body parts together. That just sounded weird. But, that made the paper flow a little better than four parts just stapled together. It required me to know enough about the subject in order to make the tie-ins flow smoothly and make sense.
Seriously, I'm more of a behind the scenes guy, than the main course...I mean attraction.
If you never pause to fumble for the right words to say, never "uh, umm, err" in the middle of sentences, never have to stare at a slide to remember where you are in a presentation, then rehearsal is less likely to help you.
Those were some of the first things my Comm 101 instructor told us to avoid doing like the plague.
I counted the number of "ums" in one poor lady's 15 minute presentation once. I lost count after 35.
That was when I made a conscious effort to not utter a vocal pause in my presentations.
Also, I found that it helped if I had a projector hooked up to my laptop. That way, I can glance at the slide that the audience it looking at, while not turning my back to the audience. That gives the appearance that you know what you're doing.
Rule one was keep eye contact with the audience. Rule two was don't use "um, uh, or er".
I'm not saying that I'm perfect. Far from it. I just know a few little things that I probably don't practice as much as I should.
Not to mention as I said in an earlier post, I routinely have to give presentations at the drop of a dime because the original presenter either chickens out or not available. And I still can give good presentations even in those conditions because I know the information.
My Comm 101 instructor just loved to pass the "hat of topics" and have us do a 5 minute speech on a random topic. Whether we knew anything about the subject or not.
I absolutely hated that part of the class. I guess I got an early start on my BSing in that class.
She also loved going to Toastmaster meetings.