Placing implants after a mastectomy is different from placing implants for cosmetic augmentation. When people have breast augmentation, the skin and tissue are left intact. These healthy tissues are capable of stretching to accommodate the implant.
During a mastectomy, the breast surgeon removes all breast tissue, leaving a very thin layer of skin. Thin skin can't easily stretch to accommodate an implant, so we use an expander to stretch it slowly and gently.