Less cable management required. Less space required - though this is less a thing with modern mouses than it was back when mice used balls versus optical tracking.
The big selling point to old farts like me - I'm 54 - is they don't cause my carpal tunnel to flare up. I can put it in an ergonomic position and I never have to move it. And while the market might not be huge, it's good to see competition. Especially for things that aid those who don't have full range of motion with their arms or hands.
Darnov