Another thing we need to realize is that this was a series ending episode, usually these episodes don't play by the same rules. Endings have a monumental job of wrapping up years of TV in a satisfying manner all while trying show why the show was great in a remembrance kind of way. Most finales aren't regular endings, they don't follow the exact same format, they don't just tell a normal narrative. Most finales bring back old cast members, touch on themes that were presented throughout the show, and try to bring closure to all these characters you love. Some finales try to do this and fail, Seinfeld is argued to be one of those. Some manage to pull it off which I think LOST fall under.
Imagine if the finale was just like any other season finale, just the story of good (Jack) vs. evil (Flocke) and two hours of them in an epic fight to the finish over the fate of the island. The story ends with just the characters who remained alive. Hey we saved the island, yay, is that it, no what, do we live happily ever after? Instead they worked the reunion aspect into the storyline of the final season, they made it fit into the narrative so that they could do a finale that does the show justice. It brought back the characters we all loved, it reminded us why we loved them, all while still telling a natural LOST story. Was it all a but convenient and cheesy, maybe, but so what it was the finale, it would feel right without everyone back, without an ending to the characters.