The Spring Data MongoDB project was intended to introduce the concepts of Spring Data repositories to enable the easy development of MongoDB repositories. It provides an abstraction layer on top of MongoDB to successfully store, retrieve, and modify documents available in MongoDB transparently. 

Spring Data MongoDB simplifies Create, Retrieve, Update, and Delete (CRUD) operations by providing the ReactiveMongoRepository interface, which extends from ReactiveCrudRepository. This hides the complexities of plain MongoDB implementations, which need to be implemented and tested by developers. Using Spring Data MongoDB could reduce development time dramatically because of this interface. 

In the coming chapters, ReactiveMongoRepository along with the default methods will be used extensively to implement business logic and to write Spring Data MongoDB repositories and test them. The following sections will show you how to use a domain model that is designed using a class diagram as a base to implement Spring Data MongoDB-based documents and repositories.