Another thing to think about is when you are working with a bigger company, you may be working on multiple shows at the same time, and having a system of forms or documentation will help keep you organized, make your life easier because you do each thing the same way each time, and helps you remember which info is for which show. As the PSM for a company, I frequently have one show in performances, 2-3 in rehearsals, and another 2-3 in preproduction. Having everything written down and easy to understand means that we don't accidentally end up asking for tap shoes for an opera, or space helmets for Shakespeare. You might also find that the more shows you do (or the older you get...

the harder it is to just remember everything.
Everything everyone else has said is also very true. Life happens, and there are times when someone will have to take over for you, fill in, or just find something when you are not around, and having the show documented means that person will be able to have what they need.