FOR WHAT IS THIS GOOD ?!?!!
For virtualization (vmware, oreacle, qemu, ...) where, some times, you need more that one virtual adapter.
For a VPN (Virtual Private Network) like a secure and encrypted connection between more computers over the internet or another unsecure network like a wifi, power line, ...
For software testing and debugging and mutch more.
In the pictures below is shown, in a simple way how it works (the pictures are not my :P )