Well, it depends what you're writing. One of the horrible things about early fiction is that the writers usually insert their own interpretations of how characters talk -- including stuttering and "ums" and whatnot. Thankfully much of this is wasted on fanfic, where you know what the author was trying to emulate -- but usually it's a distraction. Great characters can get by without habits written into their dialogue.

If you restrict writing to only the "sounds" words produce when they are read, good writing should sound like spoken language is still hard to defend. Besides how the brain differently processes words seen from eyes, the varied methods of reading (reading linearly, scanning, searching, etc) gives multiple definitions of "good writing", depending on the context, goal, reader, and many other factors.

If you don't, then many other kind of glyphs and grammars arise. At the extreme, diagrams, mathematical expressions, etc, often lack even a correspondence with spoken language. Similarly, spoken languages have many subtle indicators and markers (often temporal or intonation-based) that isn't easily translatable to the written word.