Is Tornado Shot a must have skill in Lightning Arrow/Lightning Rod BD?

I've seen a lot Amzon/LA+LR BD using this Tornado Shot at the begining of Pinacle Boss fight, incl Font of Rage and Mana.

And the fight is so Meta quick, I don't know why because the weapon and DPS are not very high on the panel ( like 60K only ).

Why? Because of the Amzon ascdncy passive point?

or what mechanics?

Could someone explain to me please~