Many pastors face hectic schedules and often the thing that is neglected is time for personal study and formation. Preaching through a whole book of the Bible provides opportunity for me to immerse myself in the text of a single book for several months. It gives me the opportunity to really consider the flow of a single biblical document, and provides extended time to reflect on the meaning of the text for its original hearers and the application of the text for its modern hearers. Perhaps most importantly, such a method of preaching provides opportunity to be shaped by the text. If your congregation can see that the text has said something important to you, then they will be more likely to listen to what it says to them.
Weekly sermon prep time becomes continuing education for the pastor who takes seriously the task of preparing to preach an entire book of the Bible. By reading several reliable commentaries as he moves through the book, the preacher can integrate life long education into his daily work. A couple of good commentaries will introduce the attentive pastor to translation difficulties and possible interpretations of key texts. Also, consulting commentaries will provide accountability for what the pastor says in the pulpit on Sunday. Over the course of his ministry, the dedicated preacher can develop quite a handle on a significant amount of biblical text in its original cultural context. This will provide the preacher with a stronger ability to apply the text to his own contemporary cultural context.
This, for me, is one of the greatest benefits of preaching expositionally. I get to read, read, read. I get to study the Bible and about the Bible and about the world in which the Bible was written. This type of study gives me the chance make a habit of life long learning. If we preach only the texts we know, we will never grow. Preaching through whole books of the Bible forces us to wrestle with difficult and unfamiliar texts. It forces us to deal with hard issues. Such preaching provides outstanding opportunity to grow both as a Christian and as a preacher.