When PvP MOBAs give players smartcast controls, this increases the ability that players have to increase their skillcap when playing against opponents. Having smartcast controls makes it so that players can use their abilities without having to click in the direction that the ability will be used. This saves the player crucial seconds that they can use to leverage an advantage over opponents. This also rewards the player for knowing the ability well, and what the range is. If they have this ability memorized, then they will have a speed advantage in the matchup.
Example: Heroes of the Storm
In Heroes of the Storm, the default controls make you point and click your abilities after pressing the key. If you enable smartcast, however, the ability will shoot off in the direction of your mouse without having to click, which gives you an advantage, and instant feedback versus your opponent.