Hi Saw,
Let’s imagine we have two routers, R1 and R2. R1 has 8 interfaces. We advertise 8 subnets from R1 to R2. In this case, R2 will have 8 entries in its routing table. When one of the interfaces on R1 goes down, R2 will lose the subnet in its routing table. Depending on the routing protocol, it will act upon this, trying to find another path.
If we would advertise a summary route that matches all 8 subnets on R1 instead, then R2 would only have 1 entry in its routing table. This will save some memory. The second advantage is that when one of the interfaces on R1 goes down, it doesn’t affect the summary that R2 has in its routing table. This makes things a bit more stable.
On small networks this doesn’t matter all that much but when you think about the Internet, summarization becomes very important.
Hope this helps!
Rene