In complex systems it's often useful to be able to receive notification when rows have been modified in MySQL so that you can invalidate various external caches or indexes. For example: memcached, Sphinx, Lucene.
In the case of MySQL's default statement-based replication this can be quite tricky to do, as it would likely require an SQL parser to determine what was intended to be modified. However, this is made much simpler with Row-based Replication (something I recommend switching to).
A C++ library exists to be able to listen to MySQL replication, but what I want to demonstrate is that it is also very simple to be able to do this with mysqlbinlog:
shell> mysqlbinlog --read-from-remote-server --stop-never
--host localhost.localdomain …