We love the way rain sounds, the way it makes everything smell, and the way it cleans out the air (isn't the day after a good rain always the most beautiful?). We don't get enough of it here in California.
Why would you want to harvest rainwater? You can use it to water your yard, and in doing so it will lower your water costs and reduce water pollution by reducing storm water runoff.
Here's a little more information from the Rain Barrel Guide.
And, for a more intense crash course in rainwater harvesting (he even uses his rainwater for drinking), check out Richard Heineken's site.
• Good Question: Rainwater Harvesting?