ArcGIS Enterprise uses the concept of "Server Roles" to support workload isolation across a multi-machine deployment. Meaning, typically you have your base deployment of Portal, Hosting Server and Datastore on one or more machines. Then, you have additional servers federated with the Portal to enable extended capabilities on your Portal, such as Raster Analytics, Image Hosting, GeoAnalytics, Mission, Business Analyst, etc.
These Role Servers exists purely for workload isolation and only perform workloads associated with those roles. For example, only the Business Analyst's Geoenrichment Server will perform GeoEnrichment jobs. Only the Raster Analytics Server will perform Raster Analysis jobs. This makes sense for workload isolation but the problem is that its an incredibly inefficient utilization of hardware resources as those servers sit idle when no members of the Enterprise Deployment those servers are federated with, performing those tasks.
When discussing distributed collaboration, Esri likes to use the example of a city with multiple departments, each department having its own dedicated ArcGIS Enterprise deployment and those departments sharing select content with each other through distributed collaboration. For example, the Fire Department might share arson data with the Police Department. The Police Department might share crime data with the Mayor's Office. The Public Works department might share project information with the Police and Fire Department as well as the Mayor's Office.
While each city department has its own distinct ArcGIS Enterprise deployment, making them members of different ArcGIS Organizations, the reality is that they are all members of the same organization - the City of Anytown, USA.
So what if each of these departments has a need for some specific server role such as GeoAnalytics. Let's say the Fire and Police Department need GeoAnalytics while the Mayor's Office and Public Works department both need Business Analyst. Under the current architecture and licensing pattern, the expectation is that the City of Anytown, USA must purchase those Server Roles repeatedly, once for each organization in order to enable the capabilities across each of the organizations. That. Is. Silly.
These Server Roles should be able to federate with multiple ArcGIS Enterprise Deployments and the organizations should be able to split the licenses across those organizations however they want. In thinking about this as I write it, it might not be a federation issue but more of a distributed collaboration issue. I'm not sure which. In any case, the status quo seems unfair.
Now, the licensing for many of these servers roles is not that terrible (BA is an exception), so its not a cost play from Esri's perspective but it does cost money to run infrastructure and keep those capabilities available. The established pattern of only being able to federate those servers with a single enterprise deployment results in capacity going unused.