I first learned the single leg deadlift at the RKC Kettlebell instructor certification. I did it, appropriately, with a couple of kettlebells. It was taught as an exercise to clue people in to glute activation. For me it was more of a balancing act. My feet both evert badly and unless I do a lot of insoles-off calf work, I have a hard time with the hinge pattern of deadlifting with only one leg. But I’m working on it.
I had never thought of it as an exercise that anyone would go much heavier on, until I took a quick browse through Youtube and found videos like the one above. I’m still trying to dial it in, mainly because I find the movements I am clumsiest in have the greatest payoff once I resolve them.