Presumably the tyre can deform easier for a given pressure.
Hence if you are riding off road and hit a bump, stone twig or whatever the tyre will deform easier. Therefore you will continue moving forward rather than move slightly up and over the object.
The tubeless tyre proponents will claim this will be a greater energy saving energy lost from the increase in friction the tyre deformation will cause.
Basically the same argument as the one that was on a few weeks ago saying larger low pressure tyres were quicker off road than skinny high pressure tyres. (which is obviously true to an extent or we'd all be using road wheels/tyres off road)
Obviously this all depends on exactly what terrain you encounter and exactly where the sweet spot will be for that type of terrain.