CppCoreGuidlines R.33 Why pass `unique_ptr` by reference?

The CppCoreGuidlines rule R.33 suggests to

Take a unique_ptr<widget>& parameter to express that a function reseats the widget.

Reason Using unique_ptr in this way both documents and enforces the function call’s reseating semantics.

Note “reseat” means “making a pointer or a smart pointer refer to a different object.”

I don't understand why we should pass by reference when reseat means "making a pointer or a smart pointer refer to a different object.”

When the function's purpose is to reseat/change the underlying object the pointer is pointing to, aren't we stealing the ownership from the caller this way and therefore should pass the unique_ptr by value, hence moving it and transferring ownership?

Is there an example that explains why passing a unique_ptr by reference is recommended?