Custom IP Settings Gateway and DNS requirement

If I am operating on a closed network (not online, not operating outside of the Subnet), why do I have to enter a Gateway or DNS address? Hog won't let you set custom settings without entering something in these fields. I understand entering an unused IP for the gateway and the Google 8.8.8.8 for the DNS...but if I'm never going to use or need them why do I have to enter them in the first place?