You will read more widely and perceptively, gathering material which you can use in your speeches. You will develop your memory, as you stock it with stories, facts, and viewpoints to use in your speeches. You will learn to become a better observer of the world around you, as you stay alert for information and viewpoints that will make you a better thinker and speaker. You will come to a better understanding of yourself, as you search your own experience to help you find worthwhile things to say. You will learn a lot by watching excellent impromptu speakers.
Speakers usually use one or two minutes to think and plan, and then give a five-minute speech.
IMP usually attracts two kinds of speakers - the lazy and the very good. They don't do well, but it's nice to encounter them in rounds because you will score higher than they do. The very good are those who know how to prepare for this event and work at it. Usually quite a few people enter IMP, many because they think it's easy. It's not, if you do it well.