Why does the Boeing 787 use Cabin Air Compressors (CACs)?
Boeing is moving to a bleedless system with the 787 Dreamliner and GEnx engines. In the past engine bleed air has been used extensively for ECS (Environment Control System) e.g. cabin pressurization and air conditioning, as well as anti and de-icing, ozone removal and many other uses.

Nevertheless Boeing firmly believes that the overall efficiencies and improved reliability are a large step forward. Moving to a bleedless system will reduce the mechanical complexity of the 787 by more than 50% compared to the 767. The new system is expected to be save about 3% in fuel and 15% more efficient overall.

The move to a bleedless architecture has been the subject of much enthusiast debate e.g. here because while many advantages are apparent it's not immediately obvious that the increased electrical loads to replace the pneumatic functions isn't a moot point. Additionally, Rolls Royce wasn't willing to commit to developing a bleedless system, so the Airbus A350 XWB will continue to use bleed air pneumatic systems.

Boeing summarizes the primary bleedless architecture benefits as being:


  
  Improved fuel consumption, due to a more efficient secondary power extraction, transfer, and usage.
  Reduced maintenance costs, due to elimination of the maintenance-intensive bleed system.
  Improved reliability due to the use of modern power electronics and fewer components in the engine installation.
  Expanded range and reduced fuel consumption due to lower overall weight.
  Reduced maintenance costs and improved reliability because the architecture uses fewer parts than previous systems.
  



787 No-Bleed Systems: Saving Fuel and Enhancing Operational Efficiencies


Pneumatics have long been one of the weakest links in modern airliners. Moving to an electric system should increase reliability of a number of sub-systems, portend reduced long term maintenance costs and aids in the improvement of many components.

One example of the advantage of moving to an "electric aircraft" is to facilitate the move from hydraulic to electric braking. I think it's a great move.