The Octo Expansion is the main reason anyone should get Splatoon 2, which is blunt but ultimately is why the sequel has gotten more longevity. The Octo Expansion is the meat the series, a greater single player campaign that tests the will of seasoned players. The ultimate reward is getting to play as an Octoling, something the game had been teasing with Marina's presence as a member of Off the Hook. It also serves as a transition between Splatoon 2 and the third entry, as you can tell the shift in art style and mood point toward a grander scope yet to come. The rewards may be small but the experience and fun story are worth the price of entry.