Retaliation for that hypnotic staring green eye of yours, that's all!   grin

But, in terms of a thought-experiment,  it would seems that the two functions would do essentially the same thing, but, for True Ambience you would actually need to have the objects in the scene to create the ambient light, whereas with the lighting from the hdri images, you could 'fake it', thus saving yourself some mesh weight in the scene......
Just a guess....the hypothesis would have to be tested.

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."