The marker appears so that you are aware of its exact size in future uses despite an animation that does not lend itself well to visibly distinct borders, not to make you react in time,
And I'm saying it's gotten tiresome and it goes against what AoE markers were used for for the majority of the rest of the game. Again, I feel they should either properly show the indicator for the entirety of the charge-up period, or just not show it at all.