The addition of Seven of Nine to Voyager also meant the subtraction of Kes, the plucky Ocampan first met in "Caretaker," one of the Delta Quadrant natives welcomed into the crew (saying it like that suddenly makes me wonder what the series would have been like if there had been more than just her and Neelix at the beginning).
Kes had been the innocent character, the one who was literally the youngest and least experienced, even though in many ways she was an extraordinary member of her species (as is typical in Star Trek), looking to push the boundaries of her potential. Early on, that meant seeing what would happen when she interacted with the only other member of the crew who had even less experience, the holographic Doctor (a sum of programming and therefor to be considered "older").
Eventually, however, Kes was paired with Tuvok, who could function as more of a mentor than the Doctor (or Neelix or Tom Paris, other characters she tended to share screen time with). Like Seven during this episode with Captain Janeway, Kes finds herself pushing at her personal boundaries to a considerable degree, making the decision to leave the ship, and in the process pushing it that much closer to home.
It's an important episode on both a character and series level.
Franchise * Series * Essential * Character
Memory Alpha summary.