> it even makes sense.  if you're not going to be on the internet,
> why the heck do you need an internet address?

Because you are building an internetwork using the Internet Protocol.

Perhaps you are connecting together several networks, some
of which might also be connected to the Internet and as
a result, you need globally unique addresses. If you can't
imagine a "network which might be connected to the Internet"
then consider a company like DeutscheBank. They have hundreds
of locations connected together and one would expect that
some of the sub-networks within their corporate network
are rather secure because they handle millions of euros in
financial transactions every day. At the same time, hundreds
of DeutscheBank employees need access to the Internet through
some other sub-networks. Now imagine that the financial
transaction side of the business needs to connect to an
IP internetwork in order to transact business with other
banks and financial institutions but that this IP internetwork
is not the Internet and is not connected to the Internet.

There was a time when we could visualize the Internet as
a nice cloud which everyone connected to. Nowadays, the
picture is rather less clear as because there are many,
many international IP internetworks that are not the
public Internet. If you go back to that cloud picture,
imagine that the millions of sites connected to the cloud
form a kind of fur that surrounds the cloud. Then
imagine that there are some very thin membranes that
touch the tips of the fur but do not touch the cloud
itself. According to RFC 2050, these membranes deserve
to get registered IP addresses because they are NOT
private networks. They are internetworks.

--Michael Dillon