Some may ask, why do so many of us preach in series? There are many reasons, but here are a handful to start with:
A series of sermons has greater leverage than a solo sermon. By reinforcing and reviewing a Bible book, the series allows for the lessons to sink in and be applied. We often are too naïve in what we expect from a single sermon, but underestimate what can be achieved over time.
A series of sermons can create momentum beyond the moment. As well as the preacher reviewing what has gone before, the listeners also know what is coming and are more likely to engage with the Bible book in advance of future messages.
A series of sermons allows messages to balance each other. If a message stands alone, then its distinctives will often need to be balanced within the message, which potentially reduces the applicational impact of it. Knowing (and if necessary, stating), that a future sermon will present another side of this issue allows the present message to be preached without excessive balancing.
A series of sermons allows for longer lead time in preparation. Knowing what is coming up allows me to channel my preparation weeks or months in advance of the sermon. This is much healthier than a brief preparation phase which does not allow the sermon to work in me before it comes from me.
A series of sermons allows for overlapped or deeper exegetical work. If I have a series in one book, or in one section of a book, I can use my preparation time to really grapple with that part of the Bible. A series of six sermons in Hebrews allows me more time in studying Hebrews as a whole than six sermons from all over the canon.
This is not to suggest that series are the only way to go, or are the way to go without thought. There is much to take into account when planning a series and sermons within a series, but these are five of the reasons why I affirm the practice of preaching series of sermons.