Hey there! Guild Wars 1 perhaps? It's pretty old, and it's more or less on life support (I could be mistaken). A large part of the PvP population is actually bots, and players rely on those bots in order to achieve their PvP goals, haha. The game is kept alive by cheating, in some ways!
Completing achievements in that game unlocks reward in its' successor, Guild Wars 2. So that could be why.
Oh, and also FF14, there are plugins that allow the game to basically play by itself...a bot-like plugin to gather materials, one to craft, one to fight and one to complete dungeons solo, one to skip cutscenes and dialogue and auto-choose rewards and one to basically do the story of the game by itself. Why would you do all that? A lot of the game is tedious, but it can be rewarding, mass-crafting items can make you rich if you sell them.