In a perfect world, a writer’s brain would be like the notebooks (or laptop) he uses, capturing every brilliant idea for future sorting and elaboration. But alas, the brain doesn’t come with a “Save As” feature. It captures only what a person, for whatever reason, subconsciously or consciously deems important enough to remember.
In the moment, a writer might think that he’ll recollect a concept or phrasing, but depending on how the brain links the new concept or phrase with the writer’s existing ideas, experiences and emotions, this doesn’t always happen. The result is that when he comes back to a project later, he has a major “oh, crud” moment and realizes that his star of brilliance is destined to fade into the dark abyss of the forgotten.
Hence the importance of taking notes.
If you’re old school, you can jot down your ideas with your favorite pen and a notebook that fits in your pocket. If you prefer a technique that’s a little more contemporary, using a digital voice recorder or even a smartphone app works. There’s no right or wrong method. Just find what works for you and use your technique consistently. You might not use everything you save, but at least there’ll be something there for you to sort through when you want or need something to work on. That matters in the craft of writing, because creativity and options are inextricably linked.