Costume creates an image, a way to visually communicate much about a character, including personality and function.  Troi's costumes, until the seventh season of Star Trek: The Next Generation, reveal more shape and skin than other crew members' costumes.  Her civilian clothing distanced her from authority (plunging necklines left no room for rank pips) and marked her role as Counselor as different from all other shipboard roles.  They also emphasize her body, displaying Troi as an object.