The way I understand it, it controls the indirect lighting, so any light that bounces off other objects. So direct light hitting a wall for instant would then bounce to other objects or maybe a hidden light sources like light from underneath a door etc, so its basically what controls the global illumination as I see it.

The total amount of samples are the same (At least when looking at the render window, if thats what you mean), which I would expect as its controlled by the pixel samples as far as I know. As to what goes on behind the scene, I assume that there is an increase since each ray would have to calculate these additional diffuse bounces.

I think whats confuses me, is as you say, the amount of firefly this single setting adds, however Im not sure why I wouldn't be surprised to why it adds so many? Because logically the diffuse bounce light would have to be dimmer than the direct light source, but still it adds lots of very bright fireflies, so personally I would actually assume the opposite to be true as more effort is spend on calculating it. Obviously it makes sense that it would add more fireflies than if this setting is set to 0 which based on the description means its not being calculated, but in that case I would expect a huge difference in lighting quality.

So I did another test, where I added 3 light emitting spheres and a single non emitting sphere at the top to receive the diffuse bounces. Also I cranked up all the settings to 16 bounces for everything, except for the pixel samples which is still 15. But looking at the two images its very difficult to see a difference I think, I would assume with all the surfaces in the broken concrete, the effect of the indirect lighting would be easily seen. But honestly the only way I can see a difference is when I quickly click between the images. Which shows that the image using diffuse bounces is slightly lighter than the one without.

Test sphere

Zooming in on the sphere at the top, shows the effect of the indirect lighting a bit more clearly. In this case I have disabled the HDRI map so the only light is coming from the 3 spheres.

So the effect can be seen on the top of the sphere where light from the roof bounces unto it.

So it have an effect, but I thought of feel like something is not 100% correct here with this setting, it might be my tests that are not good enough to clearly show the effect of it. But taking into account that leaving this setting above 0 will double render times with very little to show for it. As it can be seen in the small thumbnails where the HDRI have been turned back on again, the effect of the indirect lighting is very easily removed, if just a slight amount of light interferes with it.