So, you're reading along, and everything's pretty straightforward. The narrator's telling us the story of the del Valle family like she's a psychic fly on the wall – she's uninvolved, but she can get inside everyone's heads. She can also zip around in time and tell us things that are going to happen in the future. You figure the point of view is "third person omniscient," and get ready to move on.
But wait! You get a few pages into the first chapter and something weird happens. All of a sudden the word "I" starts popping up a lot in the narration, and you realize some grumpy guy is telling us his story in the first person. After a few paragraphs of this we go back to our nice, comfortable third-person narrator, but every chapter or so this dude butts in, and it's all "I, I, I." It doesn't take us long to figure out that the loud-mouthed narcissist is Esteban Trueba, but why give him his own soapbox to stand on?
Well, structurally it all makes sense when you read the Epilogue, which, by the way, is also told in the first person, but not by Esteban. Check out "What's Up with the Ending?" for an explanation of who's actually writing the whole story, and how it happens that Esteban Trueba is able to insert his own perspective into the narrative as it goes along.
Thematically it also makes sense to tell part of the story from Esteban's point of view. Yes, he's kind of a jerk, but getting inside his head helps us understand the insecurities that make him act the way he does. (For more on Esteban Trueba's inner emotional turmoil, see his "Character Analysis.") After all, if everyone thought the same way as the narrator, there'd be no conflict to speak of. And without conflict there'd be no story in the first place.