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)?

Answer

Genealogies are included because in all societies (except ours?) they were important.
You got your standing from whoever was your ancestor.
You would remember off by heart the list of your ancestors’ names – which is why we see such detail.
All your family would recite the same names, which made remembering very easy.
You would reel off your list whenever you were required to give your credentials.

The Bible is primarily a factual history book, rather than a fictional story.
If you were making up a story, you wouldn’t include lists of ancestors.
If you were writing a historical account, your readers would expect to see documentation of your claims.
That is what Genealogies are.

Of course, this enables us to study the genealogy of the Messiah, which is richly rewarding. Did you know that according to the genealogies in Matthew and Luke, Jesus was the true King of Israel, being descended twice from King David, through his mother and his legally adoptive father?

And 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.