Showing how you can read information from nearby particles to drive a deformation on a mesh.
With this, you could make droplets running down a window, or a tear running down a cheek. Possibly even to aid making a realistic ocean swell. (I'll have to try that)
As you can write to weightmaps, this could also be transferred to the shading tree too.
This does ramble on a bit and teeter on going wrong a couple of times.
However I think its quite useful and It only shows the tip of the iceberg in terms of possible applications.