Dave Elliott: KML is used by google earth and google maps(and flickr i seem to recall). If you are creating some custom maps, that say, show all the stores in a country then it is the language that all the data points will appear in, for example.(if memory serves you can also use it calculate distances from train stations, or specify loads of other different things)
Rich data interactive maps are pretty, searchable and could be good content, which is obviously a benefit!
You can also use kml as part of schema data and within your xml sitemaps, which will help to verify your local listings(and allegedly helps getting them indexed more quickly).
I'm not convinced there are any genuine ranking benefits but it is a subject i haven't delved into properly for ages.(as in before the time i knew what seo was)