Simply put, Fire Emblem: Awakening is a turn-based strategy game. But a lot of its charm comes not from the engaging gameplay, or compelling storyline, but off-the-battlefield interactions between characters. Strangers evolve into friends (even begrudgingly), and sometimes marry (by Fire Emblem: Awakening's strict definition of 'marriage'), eventually ending up with a child or two.
The little scenes between characters are pretty amusing, and it's physically impossible to see all of them in one game. There is even arguably a strategy to choosing what abilities you want characters' children to have, and as such (somewhat frighteningly), who they should marry. Thus there is a lot of replayability to this installment in the series, beyond just completing the game on the different difficulty settings that Fire Emblem: Awakening provides.