Well, this isn't the best example, but it shows a reason/method of using a hint to optimize outside. I ddin't want to search too long looking for a perfect example.
By putting this one here I limited players views of the moat from the spawn area.
You are trying to limit the players view by cutting large leafs into pieces that can't be seen by other leafs. (the players 'view' as far as the engine is concerned is from within ANY point inside that leaf. So in this case even though the player is standing on the ground they might as well be floating in the top of that leaf)
In this case a player standing right next to the wall shouldn't be able to see a player on the other side of the wall. However the leaf the player is in is taller than the wall, so it can see the leaf on the other side.
This is probably a case of over optimizing, thus not a great example. But it does do the intended purpose. (over optimizing and adding too many hints can be as bad as n9ot having enough, you have to decide if cutting the leafs is going to have enough benefit to be useful or not.
You can see that after the hint was added the leaf the player is in is only as high as the wall (on each side) so players standing next to the wall wouldn't render each other (unless one jumped higher than wall).
You may wonder why the hint is there/shaped like that. It doesn't have to butt up to wall or anything. In this case I 'sealed' the top of the area inside the wall with the hint and stuck it out far enough on the other side to cut that leaf. It could be a 1x1x1 unti cube and cut a leaf that is 512x512x512.