I believe, it has also something to do with the loyalty bonus. Most players (if not all), have a clause in their contract, that entitles them to a share of the transferfee, agreed between the buying and the selling club. By submitting a written transfer request, a player is prepared to wave his share of the fee goodbye, in order to get permission to speak to the interested club.
At least, that's what I always understood it to be.
At least, that's what I always understood it to be.