As writers, we often think we should be writing every free minute we have. That makes it very difficult to have one of those things called a life. If you are able to set reasonable goals for yourself and stick to them, you won't have to work like a maniac when it comes to crunch time. Tracking will help you make sure you stay on schedule.
Another wonderful thing about tracking? It lets you know just how fast (or slow) you write. When you get a contract and they ask, can you have the book done in six months?, it's very tempting to say "of course!" But how will you know if you can do it, if you don't have some method of measuring how long--in the REAL world--it takes? Knowing in advance what your output is can save you untold stress down the road.
Hope this helps you as much as it helps me!
Instructions on how to use the tracker.