When crouched near low cover, which you're taller than the object when you're standing up normally (i.e: table, low wall, edge of trench), ADSing will make you raise your stance slightly to mount the weapon on top of that surface, instead of just raising your weapon up, like it does on regular walls.
If you just get near when without ADSing, your character will raise the weapon in a overhead blindfire stance, allowing you to just shoot over the object.
Summing it up: ADS = Mount weapon; just shooting = overhead shooting