Leading-edge high lift devices such as Kruger flaps, slats and droops reduce the stall speed and increase the stalling angle of attack. In the case of slats air is ducted from below the wing to improve boundary layer energy, delaying separation and the stall.
Also a consequence of swept-wing aircraft is that there is a spanwise component of airflow - ie: air flowing from the wing root to tip. This results in a deeper boundary layer at the wingtip and the likelihood that the tip will stall first. Bad because:
- outboard ailerons lose effectiveness
- tip stall -> loss of lift at the tip -> centre of pressure moves forward -> pitch up
Slats help to reduce this spanwise flow and the tendency for a tip-stall.