Aesthetically and for their richness of flavour. Feels more sophisticated and deeper than many common plums to my part of the world, like yellow or Doris.
Their green streaks are a beautiful complement to deep purples and blood reds - their insides are also lovely and bloody often. Tart little skins, they seem to remain more taut and less watery than many and tend to have a cute and satisfying nub at the end; heart shaped and juicy. :)