1. Nothing wrong with hand sanding, it's really a case of experience that teaches you when, where, and how to get best results.
Has safety limitations for access inside rotating pieces and broken continuity of surface, very useful for stationary treatment of rogue spots.
2. Friction driven sanding discs can be much quicker to remove material and small circular scratch marks that result are less visible to the naked eye.
Very easy to spin up far too fast, reducing the effective cutting of the abrasive, plain bearing versions rapidly fail due to grit getting in bearing, sealed ball bearings are essential for good working life. Only works whilst piece is rotating so useless for spot treatments.
3. Slow speed Drill or Flex drive driven discs can alter the approach angle to get in areas that a friction driven device can't access or won't work.
Does not rely on rotational speed of piece therefore allows better control of material removal, works with piece stationary on rogue areas, allows use with interrupted surface and natural edge pieces. If using small hand held Drill/Driver speed of cutting disc is infinitely variable to give better control of material removal, (not all flex drive systems have speed control).