The Kruger flap is a fairly basic device that folds forward from below the leading edge to divert airflow over the top surface. More mass flow over the top equals more lift. At higher angles of attack - but not very high - the flow breaks down, stalling the wing. They are usually installed inboard.
The cambered leading edge does approximately the same thing at moderate angles of attack, but hangs on, giving smooth flow and lift to much higher angles. These can be installed full span.
The Kruger flap therefore has, and had on early Boeings, the job of increasing lift and then stalling the wing root early to counter the pich up at stall inherent in swept wings.
Dick W