Okay, quickly now:
1. How long is your bowstring?
2. What string material is it made from?
3. How many strands does it have?
4. What thread is used for the end servings (and the size)?
5. How long are the end servings?
6. How big are the end loops?
7. What thread was used for the center serving (and the size)?
8. How long is the center serving?
9. Where is the center serving placed?
10. Where are your nock locators placed?
11. How many twists are in your string?
These questions need to be answered to create another bowstring for your bow. In addition there are color choices involved.
Now, were you to have a catastrophic failure of your bowstring (accidentally severing it on a sharp edge you didn’t see, for example), these are the questions you would need to answer to create a replacement. Of course, you would be well prepared and have a second shot-in bowstring with you as a back up. You do, don’t you? This is one of those lessons you don’t want to learn the hard way, but many archers do. They have to withdraw from a tournament because of an equipment failure they were not prepared to correct.
My point here is that having a written record of the details of your archery equipment in a journal you have with you is a wonderful way to store away these details for when you need them. I recommend a larger notebook in your bow case, a smaller one in your quiver.
You could keep all of those measurements and specifications in your memory, but is there any advantage in doing that? I suggest that not only is there no advantage, it can actually be a disadvantage. (Imagine that you shoot multiple bows, multiple styles, etc. Are you going to keep those details in mind and not get them mixed up?)
So, coaches, teaching the keeping of an archery notebook is an archery skill worth teaching to your serious competitive archers. It involves, of course, keeping up the notebook so that all of the information in it is accessible and current.