Normal birds generally wear down their beak naturally and do not generally require regular beak trimming. Unfortunately some birds develop abnormal beaks, this can be as a result of a developmental problem, nutritional deficiency, illness or can also be due to an injury.
In these cases the two different parts of the beak (the upper and lower) may start to miss each other, which becomes a problem in some cases as the area that is not connecting with the opposite beak then continues to grow. If this occurs then beak correction is generally recommended