Manual operation of ropes on traditional flying systems relies on counterweights to balance the weight of the load. As scenery becomes more complex (and heavy) this becomes more difficult to manage. There are also health & safety risks associated with moving heavy weights around at high level.
Powered flying uses electric motors, and can achieve far higher levels of accuracy than is possible with human-controlled flying.
Nomad system by Stage Technologies:
- Complex Automated Flying System in the Olivier Theatre, National Theatre (London)