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.
To preserve the alveolar bone
To allow proprioception from the periodontal ligament of remaining teeth
Carrier for patient to adjust
All of the above