decurion:
The flaps selected for takeoff have a direct influence on the V speeds (e.g. the rotation speed). If they don’t match with the flaps selected you can get into serious trouble. There have been a number of occurrences in which the pilots had forgotten to set the takeoff flaps correctly and decided to do so during the takeoff run (despite the takeoff config warning they got). The result was (in some of the cases I known of) that the stick shaker came on after lift-off.
I'll likely be sent to the woodshed for this: More than a few crews have forgotten to set the flaps for takeoff, then when the warning sounds off with application of takeoff power the crew wakes up and selects takeoff flaps and presses on with the takeoff.