The same is true when the flash is on the camera, facing away from the subject into empty space with nothing to reflect it back to the subject.

The point of TTL is to adjust flash power automatically under the assumption that it has an influence on the scene.

If the flash is on camera or not is not too relevant. In event photography or photojournalism for example, you can sometimes see photographers holding the flash in one hand and the camera in the other. This gives more flexibility in positioning the flash. In run and gun scenarios as those mentioned, an automatic flash that helps getting the shot is more important than achieving perfect exposure.

its batteries will be wasted as it goes to full power since 99% of its light never changes the picture.

I often bounce on-camera flash into high ceilings or walls, because I want to get soft light. This also leads to high power flashes. I rarely have battery problems.

Your thought is right: all the light that does not eventually reach the sensor is wasted, but there's no simple way to increase that efficiency.

Some systems are coupled with the autofocus to determine flash settings based on subject distance, but this works only on-camera and only without tilting or rotating the head of the speedlight.

In summary: while not achieving perfect exposure for the reasons you mentioned, TTL is still useful either on or off-camera to get a good exposure automatically.