What immediate protection do subsurface asteroid habitats offer colonists over surface living?

Protection from radiation and thermal stability

The decision to excavate into an asteroid's interior rather than living on the surface is primarily driven by environmental shielding needs. Living beneath the surface provides immediate and significant shielding against the harsh vacuum of space, but most critically, it mitigates exposure to cosmic radiation, which is a major long-term health risk for human occupants. Furthermore, the mass of the surrounding rock provides excellent thermal stability, insulating the interior living quarters from the extreme temperature swings characteristic of the asteroid's exterior exposure to direct sunlight and shadow cycles. While excavated regolith piled atop the habitat also serves as shielding, the inherent mass of the asteroid itself is utilized for both radiation defense and thermal regulation.