Why would you decide to use scale sets and not scale out?
"scale out" is a general term that describes a type of capacity expansion concentrating on the addition of new hardware resources instead of increasing the capacity of already available hardware resources.
Azure Scale Set is an Azure feature used to provide autoscaling (or "scaling out") VMs based on a recurring schedule or app performance.
If you want to scale out with VM(s) that are "identical" then use scale sets. You can’t use scale sets otherwise.