Commentaries get a lot of use from scholars trying to understand specific books and passages of scripture. Commentaries fall directly into the category of Biblical Studies.
However, commentaries are also useful in the areas of practical theology and ministry, where they are a vital resource for homiletics (preaching) and liturgy (worship). Many pastors and other clergy use commentaries to deepen their understanding and response to scripture passages in the lectionary readings. This allows them to present the texts and themes responsibly, with a broader sense of the meaning of (and controversies surrounding) a given pericope (scripture passage).