After wading through the tutorials, thanks Acadia, I think I have some things to add.

Olivier's excellent tutorial at RDNA got me thinking, so I set up a "flexible" IBL material using bagginsbill's matmatic and rolled my sleeves up:

No Lights - or 6 Lights
Folks often refer to the IBL as if it wasn't a "real" light.
However, the IBL map is often divided up into 6 areas, or directions (in fact they are infinite - but 6 is easier a lot to think of) from which the light is coming..
What people don't always get is that they can be considered as 6 seperate lights, where the area in the map defines the colour and the intensity of each.
The image posted shows the 6 areas (the outer part is all the "behind" llight) in the IBL map and the corresponding render.

This is where folks say, "That's cute" and stop, but let's go further.....

My favourite question is "Why?", my second favourite is "Why not?"