Why do so many books include such extensive genealogies, what was the OT Jewish purpose/reason they included this? (Nehimiah with the people who built things for example)?
Genealogies are included because in all other societies except ours, they were important. You got your standing from who was your ancestor. You would remember off by heart the list of your ancestors’ names – which is why we see such detail.
As the Bible is primarily a factual history book, rather than a fictional story, we would expect to see documentation of its claims – which is what Genealogies are. Of course, this enables us to study the genealogy of the Messiah, which is richly rewarding, but even the less appealing genealogies contain real gems of God’s intervention – A genealogical study of Shealtiel, Zerubbabel and a word study on the “signet ring”, for example, is deep teaching about God’s ways.