>>717217748
Two reasons
>To unlock the hard mode skirmishes you need to be in new game+
>One of the characters isn't unlockable until new game+
As a bonus, part of each character's biography is what they do after the war, which you can't see until... you guessed it! New game+
this section only unlocks if they live and you unlocked all of their potentials btw
New Game+ is much easier anyway as you retain weapons/levels/orders from the first playthrough and can replay every mission as many times as you want