One of the things Google uses to determine which businesses they’re going to show at the top of Google Local/Places is the number and frequency of listings of your business on other internet “yellow page type” websites.
Google’s primary objective is to give their visitors the best and most relevant results possible. Let’s say you have two local comapnies, one of them has a listing on Google’s website but that’s pretty much all we can find about them when we go out into the web. Compare that to another one that’s listed on Google but also has a listing on Yahoo!, Bing, Yelp, Merchant Circle, Superpages, Best of the Web, Citysearch, ect.
Having tons of listings on different websites also helps authenticate that Google has the proper information for you. If Yellowpages and Yelp and Bing all have the same address and phone number listed, it increases the reliability of the data and therefore increases the ranking.