Short Answer
It allows better visibility into decking, flashing, underlayment, previous workmanship, and hidden moisture damage.
Detailed Explanation
A full tear-off — removing all existing roofing down to the deck — provides several critical advantages over installing over existing roofing. First, it allows full inspection of the roof deck for rot, damage, or deterioration that would otherwise remain hidden. Second, it allows replacement of deteriorated or improperly installed flashing that can't be properly evaluated or replaced with existing roofing in place. Third, it allows installation of new underlayment and ice-and-water membrane directly on a clean deck. Fourth, it reveals any previous workmanship problems or hidden moisture damage. Fifth, it ensures the new roofing system is installed on a flat, clean, sound surface, which is essential for proper shingle performance and warranty compliance. While tear-off adds cost, it provides the foundation for a roof that performs as intended for its full service life.
Why It Matters
A full tear-off is the foundation of a proper roof replacement. Skipping it to save money compromises the entire new system.