Why does unique_ptr have a nullptr_t constructor?

Question

It isn't clear to me what the benefits are.

If I have:

Foo* foo = nullptr;
std::unique_ptr<Foo> unique_foo(foo);

Is the nullptr_t constructor called in that situation? Or only if you do:

std::unique_ptr<Foo> unique_foo(nullptr);

Thanks!

There is some discussion here which is to allow you to passing in nullptr_t, otherwise it won't compile since it won't cast to type pointer. So my question may be why it doesn't cast?