This isn't some made up filler for the show, if you've finished the main story in the game it only makes sense to play the flashback chapter 'Left Behind'. This explores Elle and Riley's friendship throughout the apocalyptic world, a treasure of wholesome connections that don't come into light anymore. A very important part of the game's overall development, happiness in it's heart from a world turned upsidedown.
I'm not saying this is the best episode, far from it. Although i will say it has major relevance to Ellie's journey. It may confuse a lot of people, mainly those who haven't even played the game. It does seem out of place but it's a flashback to how Ellie got infected. It is important, but could it have also worked as a short 20 minute segment? Maybe, although it still doesn't make it unnecessary.
I'm not saying this is the best episode, far from it. Although i will say it has major relevance to Ellie's journey. It may confuse a lot of people, mainly those who haven't even played the game. It does seem out of place but it's a flashback to how Ellie got infected. It is important, but could it have also worked as a short 20 minute segment? Maybe, although it still doesn't make it unnecessary.