Short answer is: I want to make the world a better place. But it is more complex than that 
When I wrote my first review on Google maps - It was just an opinion that I desperately wanted to share.
I had no clue that it would grow to be my hobby. A hobby, that helps me to perfect my analytical and writing skills and give the world an opportunity to learn from my experience.
Being a Local Guide is an honorable responsibility but I still take it playfully. I want my reviews to be as entertaining to read as they are genuine. I write them like I am telling a story to a friend: it has to be truthful and fun.
Parents taught me - the fool is the one who doesn’t learn from his mistakes and the mistakes of others.
Thus, one thing that inspires me to write a review - is to let another person know which expectations were met (or not) while visiting a particular location. So with that information, they can make better choices for themself and hopefully learn from my experience. For example, when we went to Rudas Baths in Budapest and I did not even think about checking their website in advance to find out later that it was a men only day in Turkish baths that I really wanted to visit…
On the other hand, I feel very inspired for a review when I see a business operating well. It brings me so much joy to engrave well deserved compliments on the expanses of Google Maps. I sleep better knowing that these dedicated and hard working people get my tiny share of support through the ratings. Once, we found this local gem in Rome, where I lived with my husband and his family. “Antica Stabia” was run just by husband and wife in their 60’s. He cooked in front of us the best pizza we have tried in Rome… The yeast culture was centuries old and passed from one generation to another. He was even happy to share with us some dough at the end of the day, because the man of his standards will never feed customers with crust that wasn’t made today.
In Russia we say: правда дороже золота: the truth is more valuable than gold. And as a Local Guide I believe there is no such thing as a bad review. By being uncompromisingly genuine - I give a useful insight to the business owners. For ones it is a wake up call, for others - homage to their hard work, or both.
For consumers my review is an instrument to make better choices for themselves. That is why it matters to read the data properly and check not only the most highly upvoted but also most recent reviews. We should all get more informed and actively contribute to improvement of the information on both sides.
Other than this, becoming a Local Guide means that my voice matters. No matter how small and insignificant I feel sometimes being 1 out of 7.778 billion, I still can make a difference. And it is priceless.
If more people had this belief incorporated in their daily decisions, we as a Humanity would have become more responsible with our words and actions. Because we are all connected with the invisible web and have to live together and make big important decisions. Like taking action to deal with the waste we created, labor we are exploiting, resources we are devastating… However, as long as we have more and more people realizing the power of individual responsibility and voice - we still have a chance.