Unlike piston engine a/c, jet a/c have an initial take off/climb out speed (V2) significantly lower than their best angle of climb speed (Vx)ie somewhere in the area behind the power/thrust curves.
By delaying the the rotation until a faster speed then the aircraft will have an initial climb speed that is closer to the lowest drag speed. This results in an increased climb performance due to the greater excess thrust available.
The penalty is that more runway length is required to reach this speed. Useful if there is an excess of runway length and an obstacle somewhere along the flight path that would not be cleared at the lower climb gradient achieved at a lower initial climb out speed.
The principle can also be used to trade off some of this 'extra' available climb performance into additional take-off weight, until minimim allowed climb performance occurs ie sufficient to meet 1 inop or obstacle clearance requirements.
[ 08 August 2001: Message edited by: Tinstaafl ]