The Book of Second Samuel takes us back to a strange time, when everything was up for grabs and it was "hip to be square." No, not the '80s (although Absalom may have very well had big hair). We're talking about that swingin' era, the circa 1000 B.C. era.
During this time period, Judah was (apparently) getting it together, becoming a kingdom, defeating its enemies—the Philistines, Moabites, the Amalekites, the Arameans (and many, many more). No one really knows exactly when this happened, though there's some archaeological evidence that points toward the definite historical existence of King David. Some scholars think that Israel didn't really exist as a unified kingdom until later, and that it wasn't the military powerhouse 2 Samuel depicts it as being.
But no one really knows. And the problems of kingship are central concerns for the people who actually wrote or compiled the book as part of the "Deuteronomistic History." But the "Deuteronomistic History" was put together around four hundred years after Daniel ruled (roughly 600 B.C.), when people really were dealing with these problems with kings and the right way of ruling—whether by a single king or by judges and religious law in a more decentralized kind of state.
So, given this setting, the Second Book of Samuel provides a lot of material relevant to the "rule by a single king" argument. It provides an image of a great (if flawed) king, a shining example for the any future rulers—an ideal to look up to.