This was feedback I received from Louis Dionne during the SG14 meeting at CppCon.

The idea is that with Folly (being a unique function), there might actually be lots of use cases of different storage types (and other behaviours), and then we will get an explosion of new types (e.g. std function, move-only function, inplace function). Maybe it is best to do what std::basic_string does, where Traits are used to provide the implementation details.

So basically the standard focuses on basic_function, and then inplace function is a second proposal that builds on basic function, providing traits where memory allocation is inplace.