There are no "bad" or "useless" animals either way you look at it.
A creator would know what to create to keep balance if he created all the rest of the natural world.
Evolution allows for the dying off of irrelevant species.
So what we are left with (suspending our personal beliefs) is that the animals that annoy us are providing some function.
A deeper study would cause deeper knowledge of the animal which would reveal what exactly that function is.