Visible speech is one name for writing, and also the title of an interesting book about writing by John DeFrancis. Speech is not the only thing that writing makes visible though.
In the Harry Potter stories there is a device known as a ‘pensive’, into which wizards can empty their memories and peruse them at their leisure. Writing acts in a similar way: it enables you to extract thoughts, ideas, memories and opinions from your head and to examine them, and also to share them with others.
Seeing your thoughts in black and white in front of you on paper or on a screen can help you to put them in some sort of order. Sometimes, in fact, you may not be quite sure what you’re thinking until you write it down.
Writing is also perhaps the closest thing we can get to giving our thoughts a tangible form. People live and die, but it is through writing that we can perhaps preserve parts of ourselves for the future world. Look at the writings left behind by the great poets and philosophers like Shakespeare and Nietzsche; they are long gone, but we are able to get a glimpse into how they thought, which has become part of the global psyche.
This is totally unrelated but I just wanted to bring it up. I noticed that there is no Omniglot entry for Proto Indo-european. I know it’s just a theoretical language but I thought it would be interesting to have on the site, as it’s very relevant to the subject of linguistics in general. Maybe it’s not there because the language (if it existed) did not have a written form.
When I write my novels I usually have no idea where my thoughts will take me. When I begin to type it is like opening the floodgates and thoughts and sayings that I would not have come up with otherwise pour out of my imagination. I believe that writing may not only be a link to one’s thoghts, I beleive that more often than not certain thoughts are born of that link.
Unrelated:
You should have a more in-depth view on the international language Ido.