For one, Lucene[1] is quite amazing. With a slight stretch of imagination, you can think of it as a document oriented database, with all fields indexed by default. Imagine loading 100M rows with 100 columns into an RDBMS and then index each column[2]. You don't necessary do that. With Lucene - and SOLR and elasticsearch for that matter - it's not even a thought. Additionally, you get to tweak your indexing with dozens of field-tested analyzers.

[1] http://lucene.apache.org/core/

[2] we do this (over 300G of complex data) with a tool called solrbulk and on a single server in less than four hours, https://github.com/miku/solrbulk