it allows you to have more that one application server running. This is a crude form of loadbalancing when used with the weight and failover when one application server is unavailable users can connect to the other one seamlessly
You can use load balancing by setting the weight for each server - if you set the weight of 1 double that of the other it will accept twice as many client connections.
You can set a client to connect to any of the servers and when a client attempts to connect HPSD will select the one which achieves a load balance - not necessarily the one you specified in client settings.
To accept clients you must also set the "accept clients" box - some large users leave one server with this clear and use it for data imports.
One last point, you can see which actual server IS being used by hovering over the icon in the bottom left corner with your mouse.