LucidSW - 2:53 am on Nov 2, 2012 (gmt 0)
There are a few reasons to do this.
One is to get more real estate on the result page. Apparently, this helps get more clicks either on PPC or natural results when you have both an ad and a regular listing. Frankly, I don't know if that's true but if you have both, you double your chances of getting a click somehow.
Second, you can get factual data from your campaign. How many people did search for that term? PPC will tell you for sure.
Third, you can test different headlines and descriptions very easily and quickly. And again, having real data to fall on to improve your ads which could be used for SEO too.
Four, you can bid on keywords in PPC that you don't rank for in organics.
Maybe others can come up with more but these would be the biggest ones that I can think of. I do have a fifth one: in my experience, ads convert better than organics and usually get higher click rates.