Think of book trailers as movie preview. You know, those things movie companies put in front of the main feature at the theatre, or on the DVDs and BluRays that you buy. The purpose of those previews is to entice you to buy tickets to that next movie, or buy the BluRay or DVD when it comes out. A book trailer serves the same purpose.
The right book trailer creates an engaging and virtually irresistible visual interpretation of your book. People respond better to visuals than to text. They remember graphics and pictures much easier than words. If you want your book to make an unforgettable impact on your potential readers, you need a book trailer.
You need skilled video production people and voice actors: