Yea, this. Dokku was basically a single-server thing. If that box dies, your site goes down until you launch it on a new box. That might not be a huge deal for smaller sites. If my blog is down for a day, it's not a big deal.
With a cluster, if a server goes down, it can reschedule your apps on one of the other servers in the cluster (assuming that there's RAM/CPU available on another server). If you have a cluster of 3 or 5 boxes, maybe you lose one and your capacity is slightly diminished, but your apps still run. If your database is replicated between servers, another box in the cluster can be promoted to the primary and another box can spin up a new replica instance.
Dokku without a cluster makes deploys easy, but it doesn't help you handle the failure of a box.
With a cluster, if a server goes down, it can reschedule your apps on one of the other servers in the cluster (assuming that there's RAM/CPU available on another server). If you have a cluster of 3 or 5 boxes, maybe you lose one and your capacity is slightly diminished, but your apps still run. If your database is replicated between servers, another box in the cluster can be promoted to the primary and another box can spin up a new replica instance.
Dokku without a cluster makes deploys easy, but it doesn't help you handle the failure of a box.