Google then sends you a summary of how your contributions were doing. Recently they send me some E-Mail to the effect of "Awesome! Your image on <business name> has been viewed 100 thousand times!" or something like that.
It entirely turned me off the whole thing and I haven't contributed since. I thought "why am I contributing to this ad-powered product that they're monetizing for millions, for free?".
It's kind of hilarious really. If you look at their landing page the benefits you can gain include things like "...and you can also moderate our forums, for us, for free!": https://www.google.com/local/guides/benefits/
Yay!