Why might you want to use FDWs when most web applications use relation databases on the back end? RDBMSes have been around for decades and are perfectly suitable to storing data whose structure is known in advance. Relational databases allow developers to create complex queries on data from multiple tables. They are secure and flexible when it comes to retrieving structured data, and they keep data consistent.
But RDBMSes are not suitable for storing data with huge variations in record structures or many hierarchical sublevels. NoSQL database models offer more freedom in data structure, simpler management, fewer system requirements, high scalability on multiple servers, and fast performance. They allow the storage of multidimensional structures with huge amount of data. On the minus side, however, they do not always preserve data consistency.