This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Rethrowing an exception allows you to go from a more specific exception to a more general one. As an example, let's say you have some tests and a test manager. When a test fails, you have the test manager throw a TestFailedException, to signal that a test failed. However, some of the individual tests throw their own exceptions when they fail (say, FileNotFoundException). When you run those tests, you want to get all of the failure information. By rethrowing the exception, all of the exception information is retained in the stack trace, and you can go back and diagnose the problem.