One reason to have the charge up while targeting and then final casting animation after you click is that it makes things go faster. Since you need to wait the animations to finish in turn based game, you need to keep them short. By diving the spell in to two parts, you can play the first part without blocking user action, thus cutting the idle animation time in half without losing epic animations.
The first part of the animation is something like 1 second.
Is that really important to you ?
I never see that in other TB games but of course, I didn't play them all.
I just checked for Xcom 1 & 2, Wasteland 2, Solasta,The Banner Saga, Heroes of Might & Magic 7, Blackguards 2 and the new Gears Tactics.
