@vvoyer already mentioned replication. But there is another thing that couchdb is good at but most people don't use it/ know about it. It can interpret arbitrary erlang code.

Which means that you can write an erlang module and call it with couchdb show functions and have the benefit of a ready made http api for you.

A practical use of this would be to use mnesia as an in memory key-value store(for similar things that you would use redis for) and then call the functions in the module from a show function. All the benefits that you get with erlang you get with couchdb.

One of the killer features (for me at least) is user accounts management. In 3 http calls I have login-logout facility in my app ready. I can't tell you how impressed some of my clients are when I show them a V1 of their product in a couple of days.

ES isn't really meant to be used as an operational datastore (although I appreciate some do use it that way). CouchDB provides strong durability and master-master replication. It's also very easy to sync changes from CouchDB (https://github.com/elasticsearch/elasticsearch-river-couchdb) to ES so they make a good complimentary pair. Use CouchDB for the raw data (where you want strong durability and write scalability) and use ES to search over it.

localDB.replicate.to(remoteDB); maybe http://pouchdb.com/guides/replication.html