Why Solandra stores Solr data in Cassandra ? Isn't solr complete solution ?

[Ertio Lew]

Solr's data is stored on the file system as a set of index files[
http://stackoverflow.com/a/7685579/530153]. Then why do we need anything
like Solandra or DataStax Enterprise Search? Isn't Solr complete solution
in itself ?  What do we need to integrate with Cassandra ?