It seems simplistic -- and it is. My writing students ask lots of questions: what timer, how, when... :-) I describe my process fully in Time to Write.
Recently I've started recommending the Pomodoro Technique to my writing students, because it answers their questions, and you can download the ebook free.
Using a timer stops you messing around. If you know you've only got 25 minutes to write something, you get on with it and write, you don't dither.
This is especially useful if you're working on many different projects every day; there's no time to muse and sneak in glances at Facebook or your email account when you're on a timer.
The key to success when you're timing your writing is organization.
Work out what you'll be writing next day the evening before. Just make a simple list.
The next day, turn on your timer and start work on Project 1 as soon as you sit down at your computer. When the timer rings, take a short break. Then turn on the timer again, and keep working on Project 1, or go on to Project 2... then take another break when the timer rings.
I've found that using a timer also ensures that writing is less stressful.
For example, if I'm writing something or other I'm not especially keen on (it happens :-)), I know that the longest I'll have to work on this horror project today is 25 minutes... you can put up with anything for 25 minutes.
And those 25 minute sessions are cumulative. Here's what I said in Time to Write:
"Time management" is exactly that; managing the time you have.
Once you've set a goal, keep going until you achieve that goal.If you do, then goal setting and achieving will become a habit, and your life will become easier for one reason: you've taken control of the only thing you can manage in your life -- yourself.
Avoid second-guessing your goals once you've set them. You'll find that if you just keep completing your tasks, that everything you do is cumulative. It's like a snowball rolling down a hill. It starts small, and then gets bigger.
Occasionally when I recommend using a timer a writer will protest that she needs to be inspired.
That's fine, but you'll find that inspiration happens when you're writing, not when you're thinking about writing.
Using a timer helps you to show up for your writing -- and showing up is all you need to do.