To set language down so it outlasts the saying. Writing is the act that makes a record possible.
Old English writan (to score, scratch, incise; to draw the shape of; to set down in writing), from Proto-Germanic *writan (to tear, scratch), cognate with Old Norse rita and Old High German rizan (to scratch, write) — the same root that gives German reißen (to tear). The oldest sense is the physical cut: writing began as carving marks into wood or stone, and the word outlasted the material it was first made in.
Language set down so it survives past the moment of saying it, in a form that can be returned to and checked rather than only remembered. A record exists here because writing does — without it, only the sound of something once spoken would remain.