The point of this is to use cheap machines with small/slow storage to coordinate client requests while dedicating the machines with the big and fast storage to doing what they do best. I found that request coordination was contributing to about half the CPU usage on our Cassandra nodes, on average. Solid state storage is quite expensive, nearly doubling the cost of typical hardware. It also means that if people have control over hardware placement within the network, they can place proxy nodes closer to the client without impacting their storage footprint or fault tolerance characteristics.
This is accomplished in Cassandra by passing the -Dcassandra.join_ring=false option when the process is started. These nodes will connect to the seeds, cache the gossip data, load the schema, and begin listening for client requests. Messages like "/x.x.x.x is now UP!" will appear on the other nodes.
There are also some more practical benefits to this. Handling client requests caused us to push the NewSize of the heap up