A scenario where a router only needs to do packet routing.

Many commercial routers have the capability to do three things: (a) packet routing; (b) fire-walling; and (c) providing VPN services.
However; in many situations a router does not need to do all three.
Describe a scenario where a router only needs to do packet routing. Please describe a scenario where a router only needs to firewall packets. Please describe a scenario where a router needs to do both (a) and (b); but not (c).