My Sister's Keeper has seven different narrators: Anna, Campbell, Sara, Brian, Jesse, Julia, and in the epilogue, Kate. Setting them apart is the fact that each of their chapters is narrated in its own font.
The reason Picoult does this is because every story has an infinite number of sides, and while our narrators might not be honest with each other (Campbell's consistent lies to Julie come to mind) we think they're being honest with us, and in hearing from each of them, we get a more complete picture of what happens.
Picoult likens this narrative style to using a bunch of different threads woven together. Like Julia's purse, for example, which has a really pretty pattern on it. Julia says, "It takes twenty spools of thread to make this pattern." And Anna responds, "Truth's like that." When Anna finally confesses that Kate wants to die, she picks at a thread on her skirt and says, "Just maybe I will unravel the whole thing" (9.5.22). No, she's not planning on flashing the court to get a mistrial, she's unraveling the truth—as much as one person can, anyway.
So, the truth is like a bunch of different threads, and each of these narrators has one. Only be weaving them all together can we get the whole story. Remove one, and the whole thing falls apart.