Google Local Guide program focuses mainly to help travelers particularly one who visit new places. If you have ever used Google map, you may already become one of the beneficiary of the program. The reviews, ratings, photos, videos etc. are contributed by Google Local Guides. A place's information is important to a traveler, especially visiting a new place. Those who wish to help others, it's one of the best online program.
On the other hand, as you asked, Google offers perks depending on one's contribution. If you reached 100000 points, you may have opportunity to get Google's products like phones / cameras / wearable or invitation to visit Google's head office, or entry to high value competitions etc. all depending on the quality of your contribution.