Slats are a necessity when the wing uses slotted flaps or Fowler flaps for maximum lift. Those flaps cannot reach over the whole span of the wing to leave room near the wingtips for ailerons.

Once the flaps are extended, they increase the incidence of the wing along the flap span. This leaves the outer wing at a lower incidence, but in the [vortex field of the inner wing][1]. This vortex field induces a high local angle of attack over the whole wingspan, which would produce a [local stall][2] at the wingtip, where the ailerons [prevent the use of slotted flaps][3]. This stall would in most cases be asymmetric: One wing tip loses lift while the other barely manages to get along with the high angle of attack. The result is a strong rolling moment which will even increase once the aircraft starts to roll, because the [local angle of attack][4] at the down-moving wing will increase further. At this condition, the ailerons are of little help, so the roll cannot be controlled.

Now consider when flaps are used most often: Close to the ground, during take-off and landing. Then imagine what happens when the airplane rolls over one wing uncontrollably. To avoid this, slats are very helpful: They give the wing an [extra stall margin][5] by shifting the stall angle up.

Aircraft with lower wing loading normally use less powerful flaps, like simple camber flaps or split flaps. Since they change circulation less, stall on the outer wing can be enough delayed by washout and reduced taper, so leading edge devices can be avoided.


  [1]: https://aviation.stackexchange.com/questions/16193/how-do-wings-generate-lift/16202?s=7%7C0.0395#16202
  [2]: https://aviation.stackexchange.com/questions/6366/what-is-the-immediate-cause-of-stall/6375?s=2%7C0.1485#6375
  [3]: https://aviation.stackexchange.com/questions/13584/why-do-airfoils-trailing-edges-point-downward/13597#13597
  [4]: https://aviation.stackexchange.com/questions/2903/how-does-stall-depend-on-angle-of-attack-but-not-speed/3620?s=1%7C0.2265#3620
  [5]: https://aviation.stackexchange.com/questions/3694/why-do-slats-deploy-first/3698#3698