Sometimes you need to get or set information between Routing Engines using a script.
One example of this could be updating utility MIB information on both Routing Engines, since no utility MIB data synchronization is provided by Junos OS.
You could open a connection using the IP address of the other Routing Engine's fxp0.0 interface, but if that link is ever physically down (for example, it's plugged into a switch that's powered off), then this just won't work.
See the page titled "Coordinating XML RPC Data Between Routing Engines" for more information.