The airbus needs to know what phase of flight it is in for most things to work as advertised (taxi, take-off to 1500ft, take-off after 1500 ft etc). All hell is let loose on the crew with myriad warnings (eg TOO LOW GEAR) when you select gear up and the plane knows it is airborne (ie not on the ground with weight on gear) but the computers think it is in another phase of flight (like cruise, approach, landing).