Probably selected off to increase performance on shorter runways. As long as you haven't just been deiced, it's probably better to leave the APU on with APU Bleed running the packs.
If using Packs OFF for takeoff, just remember to select the first one on at first power reduction.
In Long Beach (KLGB) we sometimes use the packs off or packs on APU Bleed for takeoff to keep us from having to use TOGA Power (Full Power). Flex power takeoffs (reduced thrust) ultimately save wear on the engines. Any reduction of EGT helps extend engine life.
Cheers,
AKAAB