Although Fire Release is weak to Water Release, fire jutsu can sometimes negate or overwhelm water jutsu depending on the Fire Release user's strength. Thus, the best fire jutsu can be a severe threat so Fire Release's versatility and potency cannot be understated.
Sasuke takes Amaterasu to another level by using Kagutsuchi to manipulate the black flames. Sasuke can change the shape of the flames in clever ways to amplify his offensive and defensive capabilities. For instance, he can cover his Susano'o in Amaterasu to deter anyone from fighting in close quarters. He can also fire arrows made of black flames to burn his target upon impact.
The arrows improve the range, precision, and speed with which Sasuke attacks while the flames cause extra damage. Simply put, Kagutsuchi enables Sasuke to perform a wide array of powerful techniques using the flames of Amaterasu that Itachi cannot do.