The book is set up like a journal, with dates instead of chapter titles. And at the end, Paul hints that Tangerine is his journal. When he reads through it to organize his thoughts so that he can write the statement about Erik for the police, he says, "I logged on and went back through all my journal entries, from Houston until today" (3.15.1).
The fact that Paul is telling us his version of events should make read carefully. He's got to be biased against his brother—wouldn't you be biased against someone who's as much of a meanie?
And that bias raises the question of how much we can trust his view of Erik. Sure, he's a jerk, but is he really someone to be feared? In his very first flashback, Paul blames Erik for attacking him. Even though Erik is obviously innocent, Paul says, "I can see everything. I can see things that Mom and Dad can't. Or won't" (Prologue.1.25).
Okay, that should definitely make us think. By the end of the story, we know for sure that Paul was right. But it always pays to be cautious. Just imagine if we were reading Erik's journal, instead. How would Paul come off?
A benefit of seeing everything through Paul's eyes—or glasses, as the case may be (for more on the symbolism there, check out Symbols)—is we get his interior life. We see his flashbacks. We hear his thoughts and feelings. We even hear him say that if Erik died, he would only be sorry because "Erik is a part of whatever it is that I need to remember. I don't want Erik to die and take his part of the story with him" (1.11.10).
Seeing events from Paul's point of view shows us his interior life in a way no other narrator could. How would the book look if Mrs. Fisher narrated it? By Joey? Or even by a third-person narrator?