Release 9.5 of PostgreSQL delivers many new features like upsert, new JSONB functions, new
GROUPING functions, and more. While some of these like
upsert or JSONB may be useful to many people, a number of these new features really only service edge cases. If you have the particular edge case a feature solves though then that new feature can be invaluable. RLS (Row Level Security) is one of these edge case features.
RLS does just what it says: it secures a row in a table. But, you do have to enable it for each table plus you need to commit to using database roles as a main security mechanism. That last part is the barrier but also the reason to use such a feature.
With RLS, you use the database tier to secure the data (at least for the enabled tables). Both multi-tenant tables and analytics schemas where users have general access to the database via a query tool are solid examples of when RLS makes sense.