Emission shaders allow designers to simulate light boxes and build 3 key light rigs to render better than just with a single HDRI IBL.
For this purpose it would be high amazing when in render mode we can move rotate objects like those mesh lights in the image below

this would give designers more control over scene design and prevent them from being stuck with this single HDRI approach. Also as a side not IBL has to be able to be fully tunred off!
Here is a scene how I set this up with a backdrop and three key lights and some minimal IBL for the fill light - much better results.
The pure IBL rendering below is not Fusion - I just show how those two compare.

