One advantage of directory-based cache coherence protocols is that you don't have to broadcast every memory transaction to the entire system. Like Professor Bryant says, if you have 1000 nodes in the system, not all 1000 of them have to see every transaction. Directories allow you to target nodes that have some reason to know about a given transaction. But, this advantage depends on how many processors are sharing the data.
Snooping based schemes rely on broad casting because the requesting processor has no idea about which processors owning the requested cache line.
Directory based schemes has a directory in home node recording all the infos about a cache line. So it allows point-to-point communication.