You can know a field or fields of the data while leaving the possibility of extra fields that you don't need to know. In practice if you're making a web applica

You can know a field or fields of the data while leaving the possibility of extra fields that you don't need to know. In practice if you're making a web application and dealing with JSON, it's typically very easy to work with JSON-based databases instead of using serialization functions. You can enforce the schema with various tools at the application level when needed. MongoDB also has some performance benefits over many traditional databases, and has "native" support for binary data. I personally prefer CouchDB over MongoDB for most application that are well suited to use a "NoSQL" database, but MongoDB is sometimes decent too.