The answer is simple. To gain more exposure in search results on Google Search, Maps, Google+ and on mobile devices. This not only applies to those that have a brick and mortar store. It is also helpful for those who run an online business.
Think about it. Consumers these days use Google to search for products, services and local businesses with their smartphones, tablets or laptops. By providing all the relevant business information when you sign up with Google Places, it helps Google offer a better and more accurate search result to consumers who are looking for specific categories of products and location.