Somebody explained up above, but one example is that it can be used to mitigate collisions. If you have a company merge with another, and both are using 10.0.0.

Somebody explained up above, but one example is that it can be used to mitigate collisions. If you have a company merge with another, and both are using 10.0.0.0/8 you could spend a lot of time and money renumbering or you could use unadvertised IPs to connect the networks.