Deployments manage ReplicaSets and provide declarative updates to Pods. The primary purpose of a Deployment is to ensure that the specified number of pods are running and up to date. They handle the creation and scaling of ReplicaSets, as well as rollouts and rollbacks of application versions. ReplicaSets, on the other hand, are responsible for maintaining the desired number of replicas of a pod, ensuring consistent availability and fault tolerance. Therefore, Deployments use ReplicaSets to achieve these objectives.