Air condition packs use to pressurize and control the temperature in the Airplane.
But ..the air flow for the packs comes from the engine compressors. taking this air from the engines, reducing max power available for takeoff.
On a very heavy TAKEOFF, when performances are limited,
A packs off takeoff might be needed
Must not forget to reopen packs after takeoff ...
Great takeoff and cockpit management by the F/O shko