They don't, and it is (or should be) considered bad practice for functions, in general, to take/return smart pointers. Normally these should only be used as variables or class members; functions, on the other hand, should take/return raw pointers (except some special cases) or, better yet, references (except when there is a need to check for null value).
I disagree? Functions which allocate things returning unique_ptr is much clearer than returning a raw pointer. Clear ownership being passed, as opposed to maybe just having a view into some internal buffer.
I mean, if you only view functions as being called for side effects, then yeah maybe. But if you're constructing a data pipeline, unique_ptr in and out makes a lot of sense.
