My guessing (since I don't work the ocean) is that because Oceanic Airspace is a procedural environment and as such if everybody flew their flight plan route then you will have lots of welded metal over the ocean.
An oceanic clearance is like any other clearance and is a mandatory readback to ensure that you have correctly undestood the route you are required to fly.
The TMI is there to ensure that you have the correct route for that NAT track.