It's also useful when you want to quickly build a "good enough" version of a feature like search so you can get it in front of your users fast and iterate on their feedback. Most of the time, they'd be quite happy with the results and you don't have to spend time on something like managing Elasticsearch.
I wrote a post on how you can use postgres to add search capabilities with support for queries like
jaguar speed -car
ipad OR iphone
"chocolate chip" recipe
http://www.sheshbabu.com/posts/minimal-viable-search-using-p...
That said, I am an unrepentant yak shaver, and there is a lot to be said in just writing those things when you need it but, Postgres would be perfect for rapid prototyping in this way.