In the world of Fairy Tail there are still beasts and monsters that pose a threat. Her role isn't limited to hunting monsters, obviously as the story starts up she'll be dragged into it along with everyone else. She's just established as being very well suited to killing beasts. If any missions do arise where a monster needs to be dealt with, she'll most likely want to participate in them as that's what she's skilled in.

While hunting beasts at the moment is her primary motivation, she will become accustom to the guild and perhaps form new motivations such as the desire to protect her comrades.