My last department did trench rescue and the training was always in a live trench. To be fair we don't really have a county fire school so we have no where to erect a concrete trench.
Live - To practice building your own trench box for responders.
This Jib Arm evolution was done to simulate a wide trench for which your tripod would not cover the footprint. Yes I know you can use an aerial ladder for the lift but there are times when you can't get one into the construction area due to unstable earth, access limitations (equipment) or underground construction being done off the beaten path like a pipeline in the woods.