It's a very minor thing.
Since the auto-battle uses magic,
it's necessary to display the amount of mana spent after the auto-battle.
Because:
The results of the auto-battle might be without losses,
but for a Dark Elf (and not only), it's also important to conserve mana.
So, you have to replay all the battles, even those without losses, because you don't know how much mana the computer spent. :)
But if it were visible, you could avoid replaying some of them. :)
This would save time for more interesting moments in the game.