>*  what is the real purpose of the network subcommand of
>   router commands?  When do I not want to include a network
>   I know about?

The real purpose of the 'network' sub-command of the router commands is to
indicate what networks that this router is connected to are to be
advertised in the indicated routing protocol or protocol domain. For
example, if OSPF and EIGRP are configured, some subnets may be advertised
in one and some in the other. The network command enables one to do this.

An example of such a case is a secure subnet. Imagine the case where a set
of subnets are permitted to communicate within a campus, but one of the
buildings is intended to be inaccessible from the outside. By placing the
secure subnet in its own network number and not advertising the number, the
subnet is enabled to communicate with other subnets on the same router, but
is unreachable from any other router, barring static routes. This can be
extended by using a different routing protocol or routing protocol domain
for the secure network; subnets on the various routers within the secure
domain are mutually reachable, and routes from the non-secure domain may be
leaked into the secure domain, but the secure domain is invisible to the
outside world.