Comment by ButlerianJihad

Back in the mid-1990s, I was working at an ISP and also at a consulting firm. The consulting firm's #1 business at the time was to get businesses connected to the Internet, where these businesses already had significant LAN buildouts.

Now these were the days before IANA had officially assigned those "Private IP Network" numbers. Nobody had any private IPv4 space to work with! So the choices consisted of: make up some numbers and hope they don't conflict, or go ahead and register your IPv4 space and get public netblocks assigned, even if you're just using them privately.

So, needless to say, we encountered some bonkers configurations, and a lot of our job was undoing some really awful configurations in order to make them compliant with actual Internet connectivity, and so that different office LANs would interoperate properly.

We were also big advocates of security, firewalls, and the venerable "DMZ/Bastion" setups from back in the day, so those Private IP Network assignments would've been really useful, along with NAT, but we simply didn't have those tools at our disposal, and our clients were basically registering huge IPv4 blocks that they really didn't need to ever use.