Due to the volatility of hard disks from their moving parts and low tolerances, some sectors become defective.
Defective sectors can even come on hard disks from the factory, so most disks incorporate a bad-block recovery system to help cope with these issues.
During a low-level format, defect lists are populated, which store a list of bad sectors, which are then mapped and a sector slipping algorithm is utilized.
The drive will skip over a bad sector using the time it would have used to read it.
Accuracy of programs, reliant on static knowledge of cylinders and block positions will be compromised, however.