>>23578558
It does have a mid recreation of the story and setting your waifus on the home screen, but the real draw was the event stories set in a near copy of the anime continuity except they dug up a relic that allows for travel between parallel worlds. Very FGO-coded except for the fact that the worlds continue to be revisited over and over for the entire runtime of the game (6.5 years) instead of vanishing which allows for some character arcs spanning multiple years. It also continues to have some of that edge and interpersonal drama found in the show.
For example, instead of just writing the game continuity as having Kanade and Serena being alive to have more characters for the gacha system, they wrote it so that they are from worlds where Tsubasa and Maria died instead, and rather than everyone being extremely happy they get to meet their dead loved one again, they realize the entire situation is very fucked up because that's not actually them ("the real Tsubasa in heaven will judge me if I get attached to this one", "We're not even real sisters, how can I pretend")
It's very fan-fiction coded but has very much gained a cult following among a large part of the fanbase. You'll see fanart of game original characters or outfits to this day despite the game being dead because there was so much content in there and we don't exactly have much else going on since it ended.
Even the ongoing cooking manga is being written by a guy that was addicted to the game and keeps referencing it in various ways.

They also made 3 CDs worth of game original vocal songs, so that helps too.