I’ll take a stab at it. The Rosary is contemplative prayer, for deepening our relationship with God - yet it can be offered for some purpose other than the self. But, it’s really a two-fer, since you cannot offer it even for others without personally benefiting from it in the spiritual realm.
The Chaplet is a prayer of petition - we are begging God’s mercy for a specific (or even general) intention. Yet, the introductory prayers and our petitions also deepen our relationship with God.
Same Rosary beads - different purpose and intention, although there can be much overlap between the two prayers. Either is excellent by itself. Together, they are a 1-2 punch against evil.