"Concerning Flight" is one of those episodes that helps define what exactly Voyager thought of itself. For a lot of fans, the show was a poor substitute for Next Generation, compounded with the sin of trying to go for a premise that seemed like it should have been one thing, and because it wasn't, only made things worse by not even being what it appeared to be.
But Voyager always knew what it wanted to be, and that was an opportunity to do the basic Star Trek exploration template in a setting with a fresh set of aliens to explore, an uncharted region of space, with characters who were pioneers of their own kind. It should be no surprise, then, that Leonardo da Vinci (portrayed by John Rhys-Davies) eventually turned up as a mentor figure for Captain Janeway (originally appearing in "Scorpion, Part 1" at the end of the previous season), years before, it should be noted, either The Da Vinci Code or Rhys-Davies' appearances in the Lord of the Rings trilogy (but following his high profile role in Sliders).
Essentially, this is an episode that gives a minor supporting character his own story, well in advance of Vic Fontaine receiving similar treatment in Deep Space Nine, while at the same time giving a holographic character a chance to let loose a little, thereby continuing one of the themes of the series suggested by the presence of The Doctor (as last seen in "Revulsion" only a few episodes earlier).
This is the kind of episode that is important for someone who didn't previously understand what Voyager was doing for seven seasons to have another look at. It's not essential viewing, but it does reveal one of the show's most basic layers.
Franchise * Series * Essential * Character
Memory Alpha summary.