A rebuild is necessary when the damage to a system or structure is extensive, compromising its functionality or safety. It involves replacing major components or even reconstructing the entire entity to ensure proper performance and longevity. Repairs, on the other hand, address minor or isolated issues without fundamentally altering the system. Choosing between the two depends on the extent of damage, cost-effectiveness, and long-term viability. Rebuilds are typically favored when repairs would only offer temporary solutions or when the damage is beyond repair.