How does the author help us to "suspend our disbelief" and why is it important?

In The Tale of Desperaux, author Kate DiCamillo suspends disbelief by talking directly to the reader. In the short note that prefaces the book, DiCamillo writes,

The world is dark, and light is precious.
Come closer, dear reader.
You must trust me.
I am telling you a story

Through this preface, and by maintaining this personable voice, DiCamillo develops a trustworthy narrative tone. As the events of the book become increasingly fantastic, DiCamillo's suspension of disbelief continues through her friendly narration and her careful assurance that what is written is true.
This suspension of disbelief is particularly important in The Tale of Desperaux because of its fairy-tale characteristics. Fairytales are intentionally fantastic to the point of losing credibility of being plausible. In order to make The Tale of Desperaux particularly riveting, exciting, and interesting from a narrative standpoint, it's important to distance the work from the excessively fictional tendencies of fairytales, and DiCamillo takes care in doing just that.