Why do I need Boost.SmartPtr for the C++ compiler that supports C++11 and later?

The boost C++ library is a famous sandbox for the language and Standard Library features that absorbed with each new version of the Standard C++. However boost components that eventually became a part of the Standard are still present in boost. One of the classic examples of said above are smart pointers. So why do I need Boost.SmartPtr for the C++ compiler that supports C++11 and later?