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.