no monitor uses ACES so it always needs to be converted to P3 to even see what you're doing right?

or you are saying if there is some intermediate transform that makes color go beyond P3 it will get clipped? then I understand...

Yeah, like, let’s say that in your compositing workflow you increase exposure then decrease brightness. If your working color space is too small, your highlights will clip when you increase exposure, then all land flat at the same level when you decrease brightness. If your working space is bigger than the gamut people can see, but your last step is to tone map into Display P3, you’ll appreciate the non-clipped highlights, even if your eyes could never comprehend what they looked like in the post-exposure-boost-pre-brightness-drop phase of the pipeline.

> or you are saying if there is some intermediate transform that makes color go beyond P3 it will get clipped?

Exactly! The conversion between ACES (or any working color space) and the display color space benefits from manual tweaking to preserve artistic intent.

The key point is that your ray tracing color space and your display color space don't need to be the same thing. Even if your monitor only displays SRGB colors, it still can be useful to have more pure primaries in your rendering system.