In networks such as those created in the labs and in small to medium sized enterprises, route filtering does not provide much benefit. This is because these networks are small, and OSPF databases, and the resulting routing table entries will be limited in size.
However, when an OSPF area reaches 60 or 70 routers, with several hundred, or even a thousand subnets, and when that area is interconnected with two or three other such OSPF areas, the routing tables, and the OSPF databases can get very large (hundreds of entries in the routing table alone!). This means that a lot of CPU and memory resources are being used to maintain them, and when a change occurs, a lot of resources are necessary to get the topology to reconverge. This taxing of resources can be problematic on such large networks.
The primary purpose of filtering routes is so that we can streamline the information found within each OSPF router to improve the efficient use of device resources. Now it should be stated here that a distribute list will only filter routes from being placed within the routing table, it does not filter the actual LSAs. LSAs can only be filtered between areas. You can see this in the lesson from the fact that the route in question is still found in the OSPF database, but it is not installed in the routing table.
Remember that when a router routes a packet, it will go through the whole routing table sequentially to find a match. This is done for each packet. If the routing table is smaller, needless to say, routing will be more efficient. (Now there are features that make routing even faster such as fast switching, and CEF, but even so, a smaller routing table is always a good thing!!)
By filtering unnecessary routes, and by providing a smaller routing table, fewer resources are used to run OSPF and to route each individual packet.
Now which routes you will filter depends upon your topology. There are many cases where routes are learned via OSPF which will never be used by a particular router, so these can be filtered.
I hope this has been helpful!