Rolling resistance, surely?
Why? What difference would it make? I’ve heard people talk about less friction between tyre and tube, but frankly I think this is nonsense. If it were the case, your innertubes would look shiny after prolonged use – mine tend to show an imprint of the inside of the tyre suggesting there’s no movement.
I’ve heard people say that they feel more like tubs, which “apparently” roll faster than clinchers. Again, I think this is tosh as tubs have an innertube, it’s just enclosed within a stitched tyre!
The only reason tubs seem to roll quicker is that the whole rim/tub combo is lighter than a clincher.
And yes, just looking at the weight of tubeless tyres, the advantages are starting to look less pronounced!