Kick was one of the basic skills or abilities possessed by all characters in Diablo 2. Kick was removed during the development cycle, shortly before the beta test, and does not exist in the final game. Characters still use the animation to kick open barrels and some other objects, but never against enemies. (Aside from some of the Assassin's special skill attacks.)
Function
Kick worked worked a bit like the Paladin's Smite skill, knocking back, but not stunning, a single target. It was meant to provide more variety in gameplay, and to be paired with some ranged attacks, which were not going to work at melee range. Bows, for instance, did not work against point blank targets during early development of Diablo 2, and targets to be shot with a bow would have had to be kicked back first.
This play style evidently proved non-viable during play testing, as the pace of the game and number of enemies increased, and bows were made to work at any range. With that change, and other modifications to how the game played, Kick eventually came to be largely irrelevant, and it was removed. Without Kick, items or skills that grant knockback take on greater importance.