The host should be powered on to help provide more capacity for the cluster or help hosts that are overloaded.

The recommendation might be made because:

The cluster is a DRS-HA cluster. Additional powered-on hosts are needed in order to provide more failover capability.

Some hosts are overloaded and virtual machines on currently powered-on hosts can be moved to hosts in standby mode to balance the load.

The capacity is needed to meet the MinPoweredOn{Cpu|Memory}Capacity advanced options.