Hi Ronny,
unfortunately the terminology used in the documentation and product is confusing. The cluster that you see, which is used for job offloading, is actually based on Sling and is not clustering at the JCR level, as one saw in 5.x or MongoDB clustering as in 6(Although Sling Topologies/clusters are used for job management with MongoDB)
From the documentation [0], the clustering you are seeing can be described as follows:
Topologies are loosely-coupled Experience Manager clusters that are participating in offloading. A cluster consists of one or more Experience Manager server instances (a single instance is considered as a cluster).
So technically this is clustering at the Sling level, so you can have loosely-coupled instances, in this way you can distribute jobs to instances in the "cluster", this is the idea behind job offloading. This setup is also required in MongoDB to ensure jobs only run on a single instance. As for the offloading agent, I just disable this when I install a new AEM instance.
Hope this helps.
Regards,
Opkar
[0] https://docs.adobe.com/docs/en/aem/6-2/deploy/configuring/offloading.html#Administering%20Topologies
[1]http://sling.apache.org/documentation/bundles/discovery-api-and-impl.html#topology-topologyview