Gaines never lets us know exactly why, but he makes it pretty clear from the very beginning that Rufe has some pull among the folks there at Marshall. That's the reason, it just so happens, that Candy mentions him to Snookum by name when she tells him to round up some folks for her and get them all out to Mathu's place (1.19).
What makes Rufe so special, though, isn't anything he says or does himself, but what his perspective in the novel shows us about other characters. His chapter is the one where Gable, Beulah, and so many other characters tell Mapes—and us—about the loved ones they've lost and the hard lives they've led. It's one of the most important chapters in the whole novel because of that.