HTC Vive controllers are sometimes waved around a lot, and you might accidentally hit the wall or ceiling with them. With a replacement cost of $129.99 per controller, you wouldn’t want to damage them.
It’s specially designed to avoid blocking the sensors. Here’s an interactive 3D model that shows that the Skull won’t block the wand’s sensors. (The Vive needs to see at least 3 sensors at any given time in order to track the controller properly.)
Mamut also has a stronger wrist strap called the Spine, and a microfiber cloth for keeping the controllers clean, called the Cape.
Mamut Skull is currently being crowdfunded on Kickstarter for delivery in December.