IMO the uses for a technical document (design doc or a shorter doc) is simple. If you get to the point where you can't hold every single detail about a project in your head at once then you should write a doc. Similarly, if it will take a long time (30 minutes at least) to explain to another engineer, you write a doc, to save yourself time.
IDK how you can think that you never need to write a document
IDK how you can think that you never need to write a document