nr-of-instances hocon entry for clustered router in Akka.net

Question

The following hocon example is taken from http://getakka.net/articles/clustering/cluster-routing.html to demonstrate the config of a clustered group router.

akka {
   actor{
      provider = cluster
      deployment {
        /api/myClusterGroupRouter {
          router = broadcast-group # routing strategy
          routees.paths = ["/user/api"] # path of routee on each node
          nr-of-instances = 3 # max number of total routees
          cluster {
             enabled = on
             allow-local-routees = on
             use-role = crawler
          }
        }
      }
   }
}

Why is there an entry: nr-of-instances = 3 # max number of total routees?

I thought the benefit of a clustered router is that it can detect routees coming online through the cluster gossip and automatically include them as routees for the given routing strategy. To me that means that the nr-of-instances несколько избыточно.