Why should I use document based database instead of relational database?

Question

Why should I use document based database like CouchDB instead of using relational database. Are there any typical kinds of applications or domains where the document based d

Answer 1

For stupidly storing and serving other-servers-data.

In the last couple of weeks I've been playing with a lifestream app that polls my feeds (delicious, flickr, github, twitter...) and stores them in couchdb. The beauty of couchdb is that it lets me keep the original data in its original structure with no overhead. I added a 'class' field to each document, storing the source server, and wrote a javascript render class for each source.

Generalizing, whenever your server communicates with another server a schema-less storage is best as you have no control over the schema. As a bonus, couchdb uses the native protocols of servers and clients - JSON for representation and HTTP REST for transport.