For the average player, it does nothing on its own. For developers and the competitive PvP community, it was revolutionary. It receives a 9/10 for technical impact during its prime, though it is now considered largely legacy technology replaced by Mixins.
PlayerAPI was the engine behind the "God Client" era. Famous clients like LabyMod , 5zig , and various "hacked clients" (like Wolfram or Wurst) relied heavily on PlayerAPI (or its sister mod, RenderPlayerAPI) to function. playerapi 1.8.9