Librados is the common underpinning of which other Ceph services behind the scenes. It is also possible for applications to interface directly with RADOS in ways that are not an ideal fit with the higher level RBD, RGW, or CephFS interfaces, but wish to exploit the scalability, networking, and data protection that Ceph offers instead of reinventing the wheel. There are RADOS API bindings for multiple programming languages, including C, C++, Ruby, PHP, Java, Erlang, Go, Haskell, and Python.
A potential use case is the Vaultaire Time Series Database (TSDB). Vaultaire is described as a massively scalable metrics database, in the vein of RRDtool and Whisper (part of Graphite):