Many scientists and engineers keep notebooks to jot down ideas, log data from experiments, and generally keep a written record of their thoughts and observations.
Human memory is selective and imperfect.
If you don't write it down, then you may never recall that great idea that you had for a start up company or that elegant proof for a theorem. Writing forces you to articulate your thoughts and encourages you to be more careful in making observations. Often that little bit of extra thought that goes into to translating your thoughts into prose can lead you to interesting conclusions that would be lost otherwise. Most of my notebooks are composed and stored on a computer. Having a computer record allows me to search my previous thoughts and observations.