The mind is a chaotic, confusing place. Give it sixty seconds, and it will iterate through 6 different thoughts and ideas. It will jump through places and times, as if trying to escape itself. But if you put a pen to a paper - or fingers to a keyboard - you will gain command over your thinking.
Writing improves the ability to focus on a single thing. An unfinished sentence will haunt you until you conclude it.
A completed sentence is a snapshot of your mind in all its terror and glory.
The written word is what separates man from the rest of the animal kingdom. It is what enables knowledge to permeate. It is what makes civilization possible. Writing is what makes us superhuman.
The mind is good at coming up with ideas, but it's a very unreliable place to store them. Do not let your thoughts go to waste. Do not let your thoughts run rabid and pointless. Write and climb the mountain of what is thinkable.