127.0.0.1 is localhost, aka your own machine. If is very common. I would almost say 127.0.0.1 and ::1 are always present is you loaded IP networking.
Having a loopback network interface like this allows people to use a common design where you always use the network, even on a computer without any physical network card.
Note that any IP from the 127.0.0.0/8 range is meant to be local.
I haven't seen it in many other examples in the net.
Possibly because it always already is there. For Linux you cannot build a kernel with networking without the loopback interface. It is hardcoded to be the first network interface. I remember somethign similar for windows were 127.1 was hardcoded in DLLs. No need to manually add this. No need to mention it in documentation.