Divrei HaYamim is a layer of Tenach that needs an explanation. It has very little to add, except the things that we saw: a different view of King David, and a different view on who are the Priests. It is a priestly source, written by a priest, in the beginning of Bayit Sheni.
The book is seemingly necessary to finish off a revolution, the priestly revolution. The priestly revolution merges two peoples into one, the northern kingdom and the southern kingdom, Yisrael and Yehuda. To put it simple, it wanted to merge, in modern jargon, the J1 and J2 haplotypes. This was done by a merge of the holy books of Yisrael and Yehuda, by the work of what is called the P editor of the Torah. The merge necessitates the merging of the priesthoods, the task of Divrei HaYamim.
P took, roughly, what is now Bereshit 1 to 14 from the books of Yehuda. History in Yisrael started more or less with the story of Avraham, Yitschak and Yaakov, which was largely unknown in Yehuda. The story of Yetsiat Mitsraim was taken from both Yisrael and Yehuda, as both had a version. The priestly source added a lot of material about the sons of Aharon, about the rituals they were to do. The result is what we call the Torah.
Unity is worthy cause. The merging of holy books may be defended. But the addition of lots of material is not defendable. Falsehood obviously is not the way to achieve truth. Therefore, the priestly revolution failed. It must fail.
