A route-map is a very powerfull tool that can be used for a number of purposes, mainly it is used for route-filtering, when perhaps redistributing from one protocol into another, ie ospf into bgp, and you only want some of the routes to be redistributed.
You can also use the route-map to do policy based routing, which allows you to manipulate traffic flow based on source ip address example.
Additionally route-maps can also be used in NAT scenarios when you want to specificy which source ip addresses should be NAT'd.
