Overdentures are indicated over retained roots instead of extracting them and making complete dentures because:

Advantages of overdentures: Ridge preservation Proprioception Superior patients acceptance Open palate possible Definitive vertical stop for denture base Support, stability and retention are improved Less trauma to supporting tissues Fewer post insertion problems than a conventional complete denture Conversion to complete denture Increased biting force (pacer FJ, Bowman DC. Occlusal force discrimination by denture patients. J Prosthet Dent 1975; 33: 602-9) Physiological advantage.