/*
One good reason, with respect to servlets, is that if you put
serializable beans into your web server session, your web server can
serialize them to disk if it's shutting down, and then deserialize them
back into memory when it starts up. In this way, users wouldn't lose
their sessions / logins when the server comes back up.
If there are any non-serializable beans in session, tomcat will fail to
serialize sessions to disk.
I believe it's also necessary if you want to sync sessions between
multiple servers in a cluster.
*/