The use of the “look table” is just a personal preference (well, technically, everything is in processing images… 
The look table is meant to give a certain ‘look’ (sorry for the pun); the DCP profiles shipped with RT have a “perceptual” look table that tries to compensate for hue/saturation shifts introduced by the tone curve, so it is designed to work with that. However:
- it’s unclear how this ‘perceptual’ corrections are computed. From my inspection of the dcamprof code, there seems to be a fair bit of personal taste of the dcamprof author in there (nothing wrong with this of course)
- if the auto-matched curve is significantly different from the DCP tone curve, the look table above might be somewhat “off” (or at least not work as intended)
- look tables in 3rd party DCP profiles might or might not work well with the auto-matched curve
- finally, using the curve in ‘perceptual’ mode gives more or less the same effect as the look table in the DCPs shipped with RT (the perceptual curve code is based on an old version of dcamprof)
Hope this helps