When sending messages to 127.0.0.1 (or the localhost) the internal network driver typically handles this by shortcutting a few steps.
If you have a networksniffer/protocol analyzer like wireshark, it can not see these shortcutted packets.
By using a loopback adapter, the messages get send much further through the stack, enabling programs like wireshark to capture the packets (and enabling you to analyze the packets)