This week's book giveaway is in the Agile and other Processes forum. We're giving away four copies of The Mikado Method and have Ola Ellnestam and Daniel Brolund on-line! See this thread for details.
From the Javadocs: Defines a general exception a servlet can throw when it encounters difficulty.
Basically there are any number of things that can go wrong, not just in the servlet but in any of the other classes in the servlet container. If you look closely at the exception, it is designed to contain other exceptions, so this exception is just a way of remembering other exceptions but also (I suspect) a way to indicate that the exception has been 'managed' by the container.
You could also throw servlet exceptions yourself if you didn't want to handle an exception yourself (such as an SQLException) but it would be better to handle this yourself rather than leaving it to the container.