assuming you are actually behind a router, not forwarding the port will prevent people from connecting outside your network
essentially someone from outside will connect to your router on that port, and the router won't have a rule for that port so it won't do anything with that packet (whereas when you forward the port it knows which machine inside the network to send that packet to)
also, using 127.0.0.1 won't let them connect to you - you'll need to know the IP address your machine has. Use ipconfig in a command prompt and look for "IPv4 Address" to find that out. 127.0.0.1 is strictly a loopback - accessing that IP or 'localhost' just routs the packets directly back to your computer without them ever leaving it. That's done by the actual protocol on the network card, not by your router.
also, turn online mode off in server.properties and it won't force auths on the Minecraft server, so they can login quicker and you can even play when your net's forgeted. there's no real point in having it on for a LAN server