Distributed Power Management (DPM) and Distributed Resource Scheduler (DRS) are not mandatory, but Site Recovery Manager supports both services and enabling them provides certain benefits when you use Site Recovery Manager.
DPM is a VMware feature that manages power consumption by ESX hosts. DRS is a VMware facility that manages the assignment of virtual machines to ESX hosts.
Site Recovery Manager temporarily disables DPM for the cluster and ensures that all hosts in it are powered on before recovery begins. After the recovery or test is complete, Site Recovery Manager reenables DPM for the cluster. The hosts in it are left in the running state so that DPM can power them down as needed. Site Recovery Manager registers virtual machines across the available ESX hosts in a round-robin order, to distribute the potential load as evenly as possible. Site Recovery Manager always uses DRS placement to balance the load intelligently across hosts before it powers on recovered virtual machines on the recovery site, even if DRS is disabled on the cluster. If DRS is enabled and in fully automatic mode, DRS might move other virtual machines to further balance the load across the cluster while Site Recovery Manager is powering on the recovered virtual machines. DRS continues to balance all virtual machines across the cluster after Site Recovery Manager has powered on the recovered virtual machines.