该程序的结尾返回值有什么意义？

std::istream& Sales_item::input(std::istream& in)
{
    double price;
    in >> isbn >> units_sold >> price;
    //检验是否读入成功
    if (in)
    {
        revenue = units_sold * price;
    }
    else
    {
        //读入失败：将对象复位为默认状态
        units_sold = 0;
        revenue = 0.0;
    }
    return in;
}