If the flash sensed the distance of that which you were focused on, knew what modifiers were attached, what kind of surfaces your were bouncing off of, how big the room is, etc., and altered the flash power to match that, your exposures for the subject at the plane of focus would be near perfect every time.
...but this is not how TTL flash metering works. This would be an impossible system to design for practical use. By the time you input all the variables, your shot would be gone.
You will get better flash exposures learning to adjust your flash power and/or f stop with distance than you will ever be able to get with any TTL flash meter....not to even mention incident flash meters....the best method for accurate flash exposures. They can be used in more hand held speedlight-lit situations than one would imagine (though not all of them).
E-TTL is a way to get some sort of workable exposure an average amount of the time for average people who who shoot average-toned compositions and cannot and/or will not do the above. It is not the way to get the ideal exposure.
Does this mean it has no use? Of course not! It just means that, like with any automated function, you have to be so aware of what it is doing, and always fudging it this way and that, that you are basically taking manual control of it anyhow! In this application, it can be a slight time saver, though you still need a manual thought process to get the best out of it.
It can also get you that quick, sloppy, ugly, lazy shot in a printable, though not ideal, way. IMO, this is its greatest use! Sometimes, having the freedom to not worry about being technically perfect can free you to snap pix at exactly the right time. An less-than-ideal exposure on a greatly-timed and positioned shot is better than a perfect exposure on a shot in which you missed your desired composition and timing.