I think its just suppose to like "wake you up" or something. Maybe stimulates adrenaline or something, im not sure.
But I do know, I have never seen a "bodybuilder" use them. That is more for powerlifters, maybe strongman type training.
I dont think smelling salts have any room in a purely bodybuilding routine. Or even training for that matter. I would assume they would be best used for max effort lifts or MEETS...strictly.
but who knows. Im a bodybuilder. Maybe a powerlifter or someome else will chime in on these who has first hand knowledge.