Before arin etc it was possible to request ip space and on the
form specify you would not be connecting to the Internet.
So those off net users can't complain if ARIN allocated the
same ranges on net. Not that it's worth doing so now.
V6, drive fast.
I hoped I was going to be able to resist answering this. I can't.
There are networks out there that are large, and interconnected, and using valid, assigned IP addresses, that have never been seen on a public router. Never will, either. It's more convenient to use real addresses than 1918 blocks. It works better in the DNS, and it's easier to wrap your mind around when you're working math problems about how much to delegate, and where.
Those blocks remain allocated to the original recipients. I just looked (via whois). They are all still there. I remain amazed that I have them all still memorized. I guess Alzheimer's hasn't struck yet.
It's not just a case of convenience, either. Try connecting hundreds of private networks (each running their own context of 1918 space) to a common network-based service without using globally unique addresses. Not even NAT can help you, if your centralized servers have the same addresses as the network participants. And the uniqueness requirements don't change just because this "little i" internet isn't routed on the "big I" Internet.