can derive reviews, images, videos, user content, usa email database webpages and more to help complete your local listing in their system. As in organic SEO, more is not always better. Some sites like bbb.org are valued higher by Google and being listed in their directories will go a long way to improve your rank. For uncompetitive markets, you may be able to get away with keywords in titles, descriptions, and a few made up reviews but that will simply not be enough when competiting against sites that can have a great presnce across the web in places like yelp.
Of course, this makes optimizing your listing a time consuming process and one that is generally out of your control. Google takes a long time to associate external content to your listing (I've been waiting for months) and has been known to associate incorrect information to your business listing occasionally. Also I had 3 'web sites' recently disassociated with my business listing, though they were all valid and they have yet to return. Always remember that Google's listing algorithm is rapidly changing and what works one day may be worthless the next.