It is indeed a bit complicated to explain. There are different reasons:
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to have a clear separation between tools operating in “scene referred” space and tools operating in “display referred” space, similarly to what is happening e.g. with darktable
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to have a more modular organisation of the code
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to fix (what I perceived to be) some inconsistencies in the way some tools were applied
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finally, it is likely that some changes were purely accidental
Just to be clear, though, I think there’s nothing wrong in the way things are done in RT. I just had a different opinion about some things and wanted to try some alternatives. Not being constrained by backwards compatibility (which is very important for RT) made experimenting quite easy.