It has been quite a while since I started contributing to Google Maps. Initially it was just tagging places I visited and rating them. But very soon I discovered how Google Maps is helping me in discovering little details about a place, which is not listed anywhere else in the web. For example, before visiting a restaurant, I usually take a look at ratings of that place in various sites. But when I started going through the Google Maps reviews in details, it helped me find out more nuances of that place, like which particular dish is exceptionally good and which others are not, whether parking is a problem in that place, whether the seating arrangements are good, is the place very noisy and so on. It helped me immensely to prepare myself beforehand on what to expect and what not. That made me realise the power of posting little details in review of the place. Then onwards, I also started doing the same and posting detailed reviews regularly. One thing is still missing - I am sure Google certainly has that in mind. We get ratings of the restaurants easily, but there is no option for rating individual dishes. If I decide to go for sea-food sizzlers for dinner, it is easy to find out the places nearby that offer that dish. But it is not clear which ones are serving the best quality sizzlers. A restaurant may be offering very good chicken momos, but they may be horrible at preparing Fried Prawn!! An option for rating dishes will help solving this problem.
Another thing I like to contribute in Google Maps is through answering quick questions of a place. This questions pops up specially after I visit any market or mall or hospital. Answering these questions help a lot of people to find out what to expect in that place. If I am visiting a new place and urgently need some medicines, I can find out easily whether the market nearby has a medicine shop or not. This kind of details is making Google Maps richer every day and reaching out to more and more people.
Having said all these nice things in a serious tone, primary reason I participate as local guide in Google Maps is - it is fun!! I enjoy discovering how Google Maps is tracking my movements and poking me every other day to post a review or answer questions! 
I enjoy contributing to the maps with my reviews and photos, and that gives me a feel that I am able to connect to a wider mass across the world by exchanging helpful information.