A bookmap is best suited for when you specifically want to create output in the traditional form of a book. Its template allows you to organize your content in a book format by adding, among others, chapters, parts and appendixes.
Bookmaps are not suited for output formats other than traditional book formats. If you are planning different types of output, you should consider using a regular DITA map and including book metadata by using the elements and attributes in the <topicmeta> element.
Example: You use DITA to publish a research paper, so you write your information in a bookmap and add the necessary chapters. You can now make use of, for instance, the frontmatter to add a cover and the backmatter for glossaries.