Just what it says. filter(fn, obj) is more readable as filter(fn.bind(obj)), because the former doesn't really tell us what obj is for. This exists in various parts of the library. We also have signatures where the source observable is provided in the predicate. That is supremely silly, as I can't even think of a use case there, and a simple closure would have worked (for example, every has both an thisArg and a source).