The mapper works like the other sql and mongo mappers so there's also a $ttl parameter that you can use to cache query results.
As for what it can be used for.. I guess pretty much anything that's doable with a file based database like sqlite but with a NoSQL twist to it.
It can be used as a very simple mongo but lacks most of the more advanced features of mongo.
Disclaimer: I haven't used it personally and wouldn't really recommend it for most things unless it's a really simple app with low data size.