stringstream

What is the use of stringstream to return string in this format.  I see at most places in my application

std::string  SID::to_string() const 
{
	std::stringstream short_sid_stream;
	short_sid_stream << _in << "-" << _out;
	return short_sid_stream.str();
}

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I am not sure why didn't the original developer wrote something like this.

std::string  SID::to_string() const 
{
	std::string short_sid (_in);
        short_sid += "-"; 
        short_sid += _out;
	return short_sid();
}

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Is there any specific benefit of method 1 over method 2 except saving 1 line ?