To eliminate wheel rim side wear-through on expensive rims. Conceivably no need for rim brake surface could allow for reduced rotational inertia/mass.
Better performance for someone doing frequent, extended, intense descents with no potential for overheating a tire.
Greater clearance for wide tires assuming frame can accommodate.
But to some extent they are a solution in search of a problem. I have them on one bike, not on the other, and don't much mind either way.