It's the way cabin pressure controllers are usually designed. If the cabin isn't pressurised it will not be controllable and will simply follow aircraft altitude, Less comfortable for the passengers. Also, it's best if the cabin is always at a positive differential pressure. Negative differential creates a compression force and is less structurally stable. The cabin is pressurised slightly when the throttles are opened for takeoff, and before landing pressurised to below field elevation until after touchdown, as Fred's post above describes. This way the cabin altitude is always below the aircraft altitude. There's a graph showing a typical cabin altitude profile in the FCOM, vol 2.Kevin Hall

Can the cabin doors be opened ..when pressurized. Like a landing emergency?

Can the cabin doors be opened ..when pressurized. Like a landing emergency?

yes, each aircraft has it's own max differential pressure for landing based on opening of the doors. The diff press should always be checked prior to opening doors and emergency exits. The slight pressure adds to structural integrity during landing. The cabin pressure systems will also drive the outflow valve to the full open position after touchdown. This also helps with passenger comfort as it helps equalize the pressure in your ears as the diff pressure is vented. The G5/550 has a neat system. After i load the flight plan into the FMS, the system self sets the landing altitude based on the destination field elevation. It also takes the baro settings from the captains altimeter. On taxi out if the jet passes 8kts during taxi or the throttles are pushed to takeoff, the system switches to flight mode and increases the pressure to .25 PSI. Max diff on the ground is .3 for the gulfstreams. Totally hands off and automatically switches the cabin low press warning trigger altitudes based on the landing field elevation. Due to the Pyne Stewart accident, the aircraft will automaticaly initiate a 90 degree left turn and descend and accelorate to VMO down to 14000ft if the low press system is triggered above FL400. The 550 will also turn on the autopilot and auto throttles if they are off when the emergency descent system initiates.

Very interesting.