Session replication is used in application server clusters to achieve session failover.
A user session is replicated to other machines of a cluster, every time the session data changes.
If a machine fails, the load balancer can simply send incoming requests to another server in the cluster.
The user can be sent to any server in the cluster since all machines in a cluster have a copy of the session.
Session replication may allow your application to have session failover but it may require you to have extra cost in terms of memory and network bandwidth.