Why do I write? I write because my head only has so much space. It helps me organize my thoughts and be able to clearly articulate ideas without having to do on the fly compression with my limited mental capacity. I can spend more cycles on compression so I can convey more information in a shorter amount of time, increasing the potential audience. Too much compression though shrinks your audience. Trying to strike the balance between brevity and clarity forces me to cleave away the unnecessary and leave only the meaningful. Mythology being the ultimate in meaning density.
Writing also helps me run a consistency check on my belief's. Writing it all down lets me compile the code and look for exceptions. Is there something which I believe that does not line up with my core beliefs? Is there a fault in my logic? When it is all put together is it logically consistent? It also allows me time to check how I know what I think I know. This helps the signal to noise ratio in what I'm trying to convey effectively upping the compression ratio even further.
Writing gives me a larger canvas to paint a more detailed picture with the same size brush(my cognitive limitations as a human). If I need 500 words or 500 pages writing can easily expand to fit every detail.
To concisely relay your thoughts you must look at them from many different angles to objectively describe them in such a way that people with diverse perspectives can "see" what you do. Just as with trying to describe a physical object you need to view it from many different angles. However, unlike physical objects ideas can have many more than just 3 dimensions.
Language is the medium with which we use to convey the design specification of our abstract concepts. The goal is to draw the image from your mind in the reader's mind. An analogy to the physical is that writing is an STL file which contains all the information needed to generate the code to have a machine make a physical part by removing metal. Though each machine will have different code specific for rendering the object on that machine, the end result is the same. Writing is much the same in that it gives other the blueprint for rendering the same idea even though the exact path of logic they take may not be the same they can arrive at the same final result.
Writing isn't the only way to instantiate a specific mental state. Games, environments, music, and movies are all designed to induce certain mental states. However writing is the least constrained in its ability to convey abstract and concrete ideas. Writing contains more meaningful entropy than any other medium. For example, this post is about 450 words long, and the average english vocabulary is about 40,000. The possiblities of 40,000450 ~= 102080 whereas all of the subatomic particles and photons are in the observable universe are only about 1089. Writing is an infinite space that we can never hope to map but somewhere in the infinite is plenty of things worth writing about.