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:
Flying at the Barbican
Flying at the Olivier Theatre (National Theatre)
A theatre’s flying system is a vital way of effecting quick scene changes, as the fly tower can store large pieces of set and then lower them down on to the stage very quickly. Traditional flying systems (like the one in the Lyttelton) use a series of bars, but the individually controllable point hoists in the Olivier give the crew many more options to play with, including flying performers.
More about the flying, lighting and scenic systems at the National
The Stage - Backstage: How the National reinvented theatre (October 2016)