Integration makes sense in many ways:
Customer has freedom to choose the best individual systems (HVAC, fire, CCTV, lighting, etc) from each discipline
without fear Buildings become more energy efficient and can reduce their consumption and emissions.

Buildings become more intelligent, ie: cause and effect strategy is able to share "incident" information across different
systems Energy efficiency and intelligent building controls lead to cost savings

Remote Diagnostics
With modern IT technologies the requirement to have engineers attend site on a call out is greatly reduced.  Broadband technology allows engineers and users to connect to sites remotely from anywhere in the world where an internet connection is available.  This concept also has the added benefit of a first fix basis, this is where a problem can be diagnosed remotely and then when the engineer attends site to rectify the fault they will have all the correct parts on the first visit, thus reducing maintenance call out fees.

Extended Use from Older Systems
A problem that many building managers face is the costly expense of upgrading systems within their buildings.  An example of this would be a building that has had a Trend HVAC control system installed for the last ten years, and then upgrades the building to a modern air conditioning system such as Daikin.  At this point the Trend HVAC is incapable of controlling the Daikin air conditioning units making the environment uncontrolled within the building.  Integration allows the old and new systems to intelligently communicate with each other sharing information such as temperatures, set points, unit modes and error codes.  This then allows the existing expensive HVAC system to be utilized to control and monitor the new equipment.

Global Cause & Effect
An integrated system allows a very powerful cause and effect strategy to be implemented system wide very easily.  An example of this would be an integrated HVAC system, Fire system and Door Access Control System.

In the event of a fire the integration device can easily allow the fire state to be intelligently transferred to the HVAC and door access system.  This would allow the HVAC system to shutdown all non essential plant that could cause a fire to spread such air handling units, other items such as smoke dampers can then be operated.  Similarly the door access system will be able recognise the fire state and automatically release all of the access controlled doors.  The benefit of this system is that expensive cables runs from the fire panel to all HVAC plant / door access controllers do not have to be installed and the fire protection can be carried out through one intelligent connection to each building system.

Real Time Cash Flow Forecasting
Integration not only allows building systems such as HVAC controllers and fire systems to be intelligently monitored but also other building systems such as electricity meters.  This has many benefits for estates departments allowing actual consumption to be easily monitored and logged for cross referencing not requiring personnel to go and manually read meters.  The logged data can then be made available as a datalog allowing a trend of consumption data to be made so as anomalies in consumption can easily be tracked.  The major benefit of this is if the control of lighting or heavy plant such as air handling units is not functioning correctly and the plant is running 24hours a day the extra power consumption can easily be identified through the logged data and the controls problem easily rectified.