They can be used for all types of stuff. Imagine two companies merging. They both used 10.100.0.0/16. If you were in one of the companies wouldn't it be nice

They can be used for all types of stuff. Imagine two companies merging. They both used 10.100.0.0/16. If you were in one of the companies wouldn't it be nice if the other re-ip'd? ... Guess what.. they won't and you don't want to. So you make a box, that turns your ips into 10.101.0.0/16 and theirs into 10.102.0.0/16... and you leave your IPs alone :). Yeah, some things won't work across that bridge. But it'll get you started. Firewalls can do really, really evil stuff. DNS? Yeah, we'll answer ALL dns queries.. Even ones you send to 8.8.8.8, or 1.1.1.1... The BOFH playbook is large, and varied. :)