Given the fact that the majority of this book is told through the point of view of a precocious, somewhat socially awkward, and certainly eccentric nine year old, already the story is severely limited in perspective. Add to this the opening conflict of Oskar's father dying in the twin towers, and this point of view is further complicated and narrowed by the potential psychological issues that Oskar might be dealing with but not necessarily talking about.
The juxtaposition of alternate points of view (one of Oskar's Grandmother and one of his Grandfather), therefore, widens the context of the story and provides background that aids in understanding Oskar's character and more about the character of his father, and is ultimately necessary to understanding the end of the book. The author does a great job of keeping each point of view almost equally limited (though different) but weaving all three together does allow the reader to see things a little bit bigger than Oskar does. Additionally, it heightens the mystery element of the plot. As Oskar seeks to solve his mystery, the reader pieces together three diverging stories comprising a broader mystery.