Several reasons. The simplest is merely the fact that the actual preached message is different from the manuscript, and often the differences are noteworthy.
There is a fascinating underlying reason, though, and it relates to the nature of preaching, and prophesying. Both John Piper and Martyn Lloyd-Jones believed that prophecy can occur in preaching when the preacher goes “off script.” They have labored in the Word, and in total reliance on the Holy Spirit have constructed a manuscript of a sermon to be preached. Nevertheless, Lloyd-Jones counseled, and Piper practices, reliance on the Holy Spirit in the pulpit, with an openness to new words and phrases that are not in the manuscript, that the Holy Spirit supplies that have particular power, insight, application and authority. This happens all the time in Piper’s sermons. Piper often goes off-script in his preaching, and these excursions are often the most insightful, powerful, sometimes entertaining, parts of his messages. How often that is prophecy, and how often it is merely Piper, I won’t attempt to assess here. This is just my reason for so highly recommending that a person listen to the audio sermon, not just read the manuscript.
Another reason for this relates to preaching versus writing, which I’m sure has been adequately developed on books on preaching, but I’ll just sketch it out here. God gave us a book, in writing, the graphe. It is authoritative, and never changes. Nevertheless, he also commanded that his people continue to preach the message, not just hand out the book. He created us with voice boxes, not just esophagi. He gave us lungs to speak with, not just to breath. He gave us tongues to enunciate, not merely to taste and swallow. He made us so that we could communicate with audible spoken words, not just thought-out-written-down words. And he commands us to continue to communicate it this way.
Writing out transcripts enables one to capture a little bit more of the audible communication. Italics can be added, all caps can be used. Nowadays we can format the transcript to more fully capture the inflections of the preached word, as well as the specific words that aren’t in the manuscript.
I still highly recommend listening to the actual sermons. I can’t recommend it highly enough! There are times that chills go down my back listening to Piper preach, that I don’t get at all from reading the later book or the manuscript online. Nevertheless, in an effort to make a little bit more of that available, I intend to keep transcribing certain messages and selections from them, Lord willing, a significant amount of his biographical message on Lloyd-Jones that I think is incredibly relevant more than 20 years later.
Stay tuned 🙂