In reference to this doc I think I understand the value of temporarily horizontally scaling a pod during an update. For example, you go from 1 pod to 2 pods - update pod 1 and then delete pod 2.
Is there any value to horizontally scaling Kubernates if you're not doing an update? Won't replicating pods simply decrease the performance of each one?
For example, doubling the number of pods while keeping the amount of RAM fixed just means that each pod will have half as much RAM.
... doubling the number of pods while keeping the amount of RAM fixed ...
I think you are mis-understanding what happens when you horizontally scale pods. Each pod is allocated a certain amount of memory and when you create new pods, each existing pod keeps using that much memory and new pods get that same amount of memory. So the overall memory usage as your horizontally scale pods is increasing linearly with the number of pods that are running.
The only constraint is when you hit the total limit of memory available in your cluster, at which point you won't be able to schedule new pods. At this point, you would need to (manually or automatically) scale the number of nodes in the cluster to add additional resources.
Is there any value to horizontally scaling Kubernates if you're not doing an update?
Yes. If you are serving traffic and the current number of pods can't handle the load, adding new pods can help. For example, during the One million requests per second demo, the author determines how many requests each nginx pod can serve and then scales the number of pods to handle the expected load. If this was a web site, the horizontal pod autoscaler could allow you to dynamically scale the number of nginx pods based on the current number of requests.