So, a bit of history to start with... XML or Extensible Markup Language is a subset of SGML that gained popularity in late 90's and beginning of the next decade. That's when SOAP was considered the best way to implement SOA and it made you use XML. That was well before JSON was ubiquitous and BSON was known to most. Software AG were the first to come up with a native XML data store called Tamino XML Server. It was rightly timed and feature rich, the open source community realized there was a need for something that offered similar functionality in the open source world. eXistDB was created.
While one might think of eXistDB as something similar to CouchDB, i.e. a document store with a RESTful API. But eXistDB has to offer a lot more in terms of application development. It allows ease of development and deeper integration into applications via support for XSLT and hence can deliver the documents in a formatted state, thus taking the processing burden off from the front end application and reduce the amount of data exchange required.
