By jotting it down, you can actually see that you are indeed making progress, and that changes to your body are actually happening! Sure they may not be as fast as you like, but progress is progress, and that's the goal. It's a motivator, it keeps you from giving up when you feel like this whole thing isn't worth it. It simultaneously gives you data to make other changes to perhaps push yourself through a plateau!
Plateau busting can be one of the most frustrating aspect of Muscle Building or Weight Loss. You get to a point where it just doesn't seem that you are getting anywhere. It can go on for days, weeks, even months! Keeping a log allows you to see things that may be causing your plateau. You can look back to when you stopped making gains or losses and see what perhaps might be causing that. Maybe you stopped doing an exercise. Maybe you started doing a little less or more time on the elliptical. Maybe you started eating a different kind of protein bar as a mid afternoon snack.
Being able to take in this data, and synthesize it into useful information to help you get to your goals is the name of the game! Big companies use this all the time, they call it analytics! But in order to analyze why things may not be working as well as you'd like you need to keep your data somewhere, and you need to keep it in a consistent manner. You don't have to use anything fancy I used a Mead Composition Book for years! You can get a 5 Pack of them here from Amazon for about $9.
So keep a log, take as many measurement as consistently as you feel doing. Using data to drive your goals is the optimum way of achieving them.