Google tries to present users with the most useful and informative search results.
The more information a search result snippet can provide, the easier it is for users to decide whether that page is relevant to their search. With rich snippets, webmasters with sites containing structured content-such as review sites or business listings-can label their content to make it clear that each labeled piece of text represents a certain type of data: for example, a restaurant name, an address, or a rating. See in details here: http://googlewebmastercentral.blogspot.com/2009/10/help-us-make-web-better-update-on-rich.html
This not only spices up your search result but also increases click rate: