They are a very usefull tool for someone that hunts hills,even on 30 yard shots.I should say especially on 30 yard shots,the steeper the angle,the bigger the difference.

Normall stand situations,they are useless but I have 1 stand that a 35 yard shot needs to be shot as a 30 and I don't hunt big hills at all,just this particular stand is on a hill.

You seem to be a bit confused as the purpose,you DO NOT shoot for the line of sight but rather for the horizontal distance because of gravity,the arrrow only covers the 30 yards in your example with gravity working against it.Gravity doesn't know there is a hill or angle there.

One more thing the angle range finder does is account for trajectory which a right triangle or pathagorean theorem does not do but that is a very slight difference.