Have you ever suspected that your house is haunted? Maybe by someone who used to live there but met some terrible fate? Okay, we know that sounds like the plot of pretty much every ghost story ever, but you can think of the Man Without a Face as the ghost that haunts the house of this novel.
See, Díaz mixes historical cruelty and fantasy evil in Wao. That's just his M.O. Yes, Trujillo is an actual person in the history books. But he's also, in Díaz's hands, a super-powerful supernatural being. This is pretty much what's going on with the Man Without a Face.
You can imagine this recurring character as actual victim of violent oppression. During a regime like Trujillo's, plenty of people disappear. And nobody ever finds out what really happened to them. People suspect. But they don't know.
It's possible that the Man Without a Face is meant to remind us of this fact: that tons of Trujillo's victims have remained nameless. Or faceless. So they haunt this novel.
And, as in any good ghost story, our deceased victim also makes others his victim. The Man Without a Face often seems like he's the one doing the evil things. For example, when Oscar gets his first beatdown at the hands of the capitán's goons, he thinks he sees the Man Without a Face above him:
Most of the time they took turns striking him, but sometimes they got into it together and there were moments Oscar was sure that he was being beaten by three men, not two, that the faceless man from in front of the colmado [gas station] was joining them. (184.108.40.206)
The Man Without a Face actually joins in on the beatdown. Ouch. Maybe now that Yunior has finally written Oscar's story down, the faceless man can rest in peace?