Welcome to the Community,
DRS is - as you already mentioned - used for live migration, which means the "source" host is still up and running and the VM's need to be live migrated to another host for any reason, e.g. Maintenence, load balancing, ... Without DRS you can still live migrate the VMs manually with vMotion.
HA comes into play if a host breaks. In this case the VM's cannot be live migrated anymore, since the source host is not reachable anymore. HA is responsible to monitor the hosts in the cluster and in case a host (or VM, depending on the configuration) fails to power on the VMs on other, surviving hosts.
André
