I want to point your attention to a great article right here: ardenpackeer.com, good stuff, deffinately worth the read.
I am looking this up today, as i ran into it last night doing more BGP labs. Again, just to re-iterate. The feature gives you the ability of taking a network, thats advertised by a certain source, and inject a more specific route of this network into the BGP table (and then to the routing table). When will this be used? For aggregation purposes.
If you receive an aggregated BGP route, and for some reason you want one of the routes it includes to be installed into the BGP table, then you can use this feature.
I wont go into the configuration detail as it is explained perfectly on the blog above. But you get to use your route-map skills.
One thing i will note however, is the fact that with the introduction of this more specific route, you can set the next hop to something else. So if you receive the aggregate route, you can use this to route for most of the traffic, while using another next-hop address for the more specific route. A nifty feature. Sort of like PBR, but with a flavor of BGP introduced.