Panning sound is key in mixing and sound placement, but amplitude is only part of how humans determine the direction of sound. The Haas effect targets another mechanism that detects small differences in time between left and right to position sound.
The Haas effect will widen the stereo of the audio by delaying the left or the right channel slightly.
Bottom line, it brings stereo width to sounds where there previously was none. Simple as that:
Haas shines on it's own, but it was primarily created to power up the different "Snapin Hosts" Kilohearts offer. These are bigger effects units that utilize all the available snapins and give you a really fun and creative workflow for combining them in any way you can think of. So have a look at Snap Heap and Multipass right now.
Haas is also available as a Rack Extension for Reason over at the Propellerhead shop.