Interior walls - why skim coat with joint compound?
I'm not sure if we're talking about the same thing with "skim coat", but it's just a higher level of drywall finishing for improved sheen management. 


  Level 1: This level means that your drywall joint tape has been
  embedded in joint compound, and nothing more.
  
  Level 2: This next level means that you have skimmed a thin coat of
  joint compound over the tape and covered the drywall screw holes. You
  can stop at this level if you intend to cover with tile.
  
  Level 3: For this stage, you apply a coat of joint compound to the
  tape and screws. Walls that will receive a heavy texture, such as
  knockdown texture, can end at this level. It would be pointless to
  progress beyond this level, since texturing is rougher than level 3.
  
  Level 4: This is the classic drywall finish.
  
  Here, you apply another coat of joint compound to the tape and screws
  and sand the dried compound.
  
  Level 5: The highest possible level of drywall finishing involves
  applying a skim coat, if applicable.
  ...
  The two instances when you need a level 5 coating: the finish will be glossy and/or light will be coming from an angle low enough to highlight bumps and depressions.


https://www.thespruce.com/the-five-levels-of-drywall-finishing-4120152

Modern joint compounds are used for their ease of application, relatively quick drying, and light weight. I'm not familiar with the application of plaster over drywall in modern scenarios. My 1950s home did have plaster over drywall, but that's a much more involved process than simply skimming wallboard.