In this chapter, more men gather to David in Ziklag and Hebron.
The fundamental purpose of this chapter is to show the unified support for David while he lived in two different cities, which exemplify two different periods in David's life: when he is fleeing from Saul and when Saul is dead and David is going to be made king. Ziklag is symbolic of David's life in exile when he is hiding from Saul, and Hebron is symbolic of the time after Saul's death when David is already reigning over Judah and about to ascend as king over all Israel.
This chapter shows that he is supported by the entire nation in both periods, but with particular emphasis on Benjamin supporting David even against their relative Saul. This chapter also shows some of the same tribal bias that was observed in the genealogy, with a particular emphasis on Benjamin and Judah as being supporters of David (particularly verses 2-7, 16). Since the genealogy seemed to indicate that Judah and Benjamin were the majority of the returning exiles in the time that Chronicles was written, it's possible that this chapter is trying to show that both Benjamin and Judah supported David, which would help smooth over any lingering discord between the tribes given that David was in conflict with the house of Saul for years (including after Saul's death).
The final section in verses 23-37 tries to present a unified nation behind David with support from every tribe, and while this is copied directly from 2 Samuel 5, it skips over 2 Samuel 2 when 1) David goes to Hebron because he is primarily supported by Judah and 2) David is almost exclusively backed by Judah, while all the other tribes support Ish-Bosheth, Saul's heir. It's only after David (and Judah) fight a war against Ish-Bosheth and defeat him in battle that the rest of the tribes come and support David.
In verses 38-40 all the people who gathered to make David king are eating and drinking together. This is a biblical motif for ratifying a covenant. We saw this happen with e.g. Israel and God eating a meal together (Exodus 24:11) to ratify the covenant and when Jacob and Laban ate a meat together to secure their agreement (Genesis 31:54). The concept of a wedding feast may itself be derived from this tradition. Here, the people are eating a meal with David to secure their covenantal agreement to obey him as lord while he protects and shepherds them.
There are more minor details that I could pick apart, but I don't find any of them very interesting so I think I'll stop here. We have learned from this chapter that Israel is unified behind David, Judah and Benjamin are best friends forever, and all Israel is filled with joy now that David is king. Life is good in Israel and we should all feel positive about it.
In the next chapter, David seeks to bring the ark of the covenant back to Jerusalem.