It's relevant to Google Places. A citation includes your business' name, address and phone/contact number (e.g. NAP). This helps spiders to index the business in its appropriate SERP category. In this way, Google visitors can go to a site exactly what they've searched for.
Hence, this is useful for businesses, especially small ones. You can use your Google Places' business citation for local search campaigns. In this way, you can rake in visitors who are likely to become your regional customers.