It is used to move assignment.
Following is the declaration for std::basic_istream::operator=.
copy (1) basic_istream& operator= (const basic_istream&) = delete; move (2) basic_istream& operator= (basic_istream&& rhs);
rhs − Another basic_istream object with the same template parameters (charT and traits).
It returns *this.
No-throw guarantee − this member function never throws exceptions.
Modifies both stream objects (*this and rhs).