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Mark Jason Dominus has a nice page [http://perl.plover.com/lt/lightning-talks.html about lightning talks], which includes this summary of why you might want to do one:
Maybe you've never given a talk before, and you'd like to start small. For a Lightning Talk, you don't need to make slides, and if you do decide to make slides, you only need to make three.
Maybe you're nervous and you're afraid you'll mess up. It's a lot easier to plan and deliver a five minute talk than it is to deliver a long talk. And if you do mess up, at least the painful part will be over quickly.
Maybe you don't have much to say. Maybe you just want to ask a question, or invite people to help you with your project, or boast about something you did, or tell a short cautionary story. These things are all interesting and worth talking about, but there might not be enough to say about them to fill up thirty minutes.
You might also like Mark Fowler's's [http://www.perl.com/pub/2004/07/30/lightningtalk.html Advice for Giving a Lightning Talk].