This makes additional lift.
I've always loved the Coaler. I don't know why, but there is something really aesthetically pleasing about it to me.
Both methods are possible.
One is internally blown flaps, where the bleed air is channelled through the wing and then vented out the back. I have a feeling that this was really only common on fighters, with a centrally-mounted engine like the F-104.
The other is externally blown flaps, which Vik was describing. Interestingly, the A380 uses this concept.
By the way - check out the stall speed on this thing;