In this chapter, we are given another, more extensive genealogy of Benjamin through the line of Saul.
I'm not sure why the genealogy of Benjamin is duplicated rather than moved here. This genealogy is much longer and more extensive than the genealogy of Benjamin in the previous chapter, and I don't know why the Chronicler wanted both. I think the purpose of this genealogy is fairly clear: it is very near the beginning of the narrative section (which starts in chapter 10), and since the narrative begins with the death of Saul, it transitions between these two segments very smoothly by concluding with the genealogy of Saul. 1 Chron 9:1 makes it clear that chapter 8 is the end of the "formal" genealogy as the author regards it, even though chapter 9 includes more names and people. Actually, chapter 9 includes another copy of the genealogy of Saul, showing once again how important it is regarded by the Chronicler.
Benjamin was the last and youngest son of Jacob, which also makes it appropriate that his genealogy be given last.
What's less clear to me is why the Chronicler wanted the shorter and less relevant genealogy of Benjamin in chapter 7. I think it might just be there because of the structure of the underlying source material. Like, maybe the original source genealogy had Issachar, Benjamin and perhaps Naphtali and the Chronicler just copied it wholesale without regards to duplicating a given tribe in the formal genealogy of the twelve tribes.
Similar to Issachar, Benjamin and Asher from the previous chapter, this genealogy of Benjamin is also possibly from a military source. The term "heads of families" occurs several times, i.e. captains, and v. 40 tells us that some of these men are brave warriors who would be capable in battle.
Verses 6-7 refer to an exile of some parts of Benjamin, but without telling us exactly which one. I'm not sure if this refers to the Assyrian exile when the northern kingdom was destroyed or the Babylonian exile when the southern kingdom was destroyed. This would actually make a pretty big difference because Benjamin was a tribe on the border between these two kingdoms so it could have swung either way and we have very little textual evidence to indicate what ultimately happened to Benjamin. In fact, I think the very existence of this genealogy may imply that Benjamin was at least partially associated with the southern kingdom, because otherwise the records would possibly not even exist for the Chronicler to have copied them. Verse 28 tells us that these chiefs of Benjamin lived in Jerusalem, which strengthens the claim that Benjamin might have been part of the southern kingdom Judah, and also explains how the Chronicler has their genealogical records.
Verse 29 begins what appears to be a separate genealogy from the passage in verses 1-27. They look separate because as far as I can tell there is no overlap between the names in one list and the names in the other list. They are both genealogies from Benjamin, though, so they are taken together as one section. The first genealogy is difficult to place in any particular timeframe because none of these names are referenced anywhere else in the bible, though v. 6-7 suggests these men could be from the exilic period.
On the other hand, v. 29-40 can be clearly dated to the early kingdom because it is a genealogy of Saul and his descendants. It includes references to some people we know, Ish-bosheth and Mephibosheth, as well as a bunch of Saul's later descendants who (like Mephibosheth) probably remained in Jerusalem.
Anyway, I don't think there's anything terribly significant about this chapter. I think it's mostly just a setup for the narrative that begins in chapter 10.