"Write for yourself" is shorthand. In part, it's to get an individual writing. After a hundred pages of short stories and essays it might evolve to a more restrictive "find your voice".
Once a writer finds a consistent voice, with recurring themes and techniques, the call to write for oneself is unnecessary. There's enough material for an editor to work with that more specific and constructive criticism can be given.
Once a writer finds a consistent voice, with recurring themes and techniques, the call to write for oneself is unnecessary. There's enough material for an editor to work with that more specific and constructive criticism can be given.