What is JMS good for?

In my opinion JMS and other message-based systems are intended to solve problems that need:

Asynchronous
communications : An application need to notify another that an event has occurred with no need to wait for a response.

Reliability
. Ensure once-and-only-once message
delivery. With your DB approach you have to "reinvent the wheel",
specially if you have several clients reading the messages.

Loose coupling
. Not all systems can communicate
using a database. So JMS is pretty good to be used in heterogeneous
environments with decoupled systems that can communicate over system
boundaries.