If you have a new idea, it opens a loop in your brain. It circulates over and over until you eventually do it or write it down. I have a paperback notebook for this. I can simply scribble down what cames into my mind, so I am free to forget it. It saves a lot of mental power.
Most todo systems are based on writing things down. You should keep a todo list, because you can’t possibly remember everything. It also helps to prioritize; you can quickly see the next item, without going through the whole list.
But todos are just one thing. What if you had a great idea? You either forget it or you keep remembering yourself to it. But neither is good. The former is like you never had the idea, and the latter consumes your mental power. Writing it down is easy and remedies both.
I am reluctant to write stuff down in a non-organized way because of the fear that my notebook will be hard to read. It’s actually a valid fear. I might save mental power by not having to remember all the stuff, but if I have hard time to find it later, it has no use.
But this fear is unwarranted. Usually, writing down everything produces a quite manageable amount of data, and you only need to seek the last few pages.
Don’t be afraid of the mess. It’s a lot less than in your brain.