Umm…again 
I guess you have to explain what you mean by “needed” colours. That surely entirely depends on personal preference and the goal of the art.
The “Reducer” add-on actually existed in BeOS ArtPaint, but was removed by one of the porters, apparently because he thought it crashed. (Doesn’t seem to now.) And maybe nobody knew about it because it was never in the docs! I personally had no real interest in it until I started thinking of turning arbitrary images into HVIF icons, via ArtPaint, autotrace, WonderBrush, and Icon-O-Matic… [And came to the conclusion it would only be a good scheme in very special cases. I successfully turned an old Amiga art of a tiger (below) into an HVIF. It is 19K!]
So just to illustrate colour reduction, here’s a JPG of the ILBM original:
and a 16-colour reduction:
That’s with the original ILBM dithering suppressed, and no dithering in ArtPaint.