The News
In the past Google used one product listing service known as Google Base. Recently, they have changed their service slightly and split it into two parts. Base is now used for real estate listings and other related listings, and, Merchant Center is now used for product listings.
Both are equally important but now separate different fields of use to make things easier for the users.
Now that you have a brief history where Google Base and Merchant Center came from, we can now address the question of why would you want to utilize either Google service.
The answer is that you want to increase your visibility to more users on the internet. Traffic via keywords and ad campaigns, well crafted content for specific key terms, link building with other sites of particular relevancy. Google Base and Merchant Center are used to submit your products, articles, real estate properties and many other types of services that you offer to the appropriate Google database for the opportunity through relevancy to be shown in search results from Google.com searches as well as any other site that uses Google's search engine to find information.
Since it would take many months for the listings to be found from googlebot and other spiders crawling your site, this acts as a sitemap for the products & services that you offer. I think of it as a quick indexer for eCommerce.
You submit a data feed to Google abiding by the rules and policies that they provide. They check your feed for errors. If you have critical errors they tell you what is wrong and what needs fixed to have an approved feed for use by Google. After you have an approved successful feed, you will see how many products are active in the Google data base and you can track which products are getting view and clicks.
At this point I do not see a down side to using Google Base or Merchant Center. If you have products and services to offer there is no reason to not submit this information to increase traffic from Google, Google Shopping, Google Maps and the other vast services Google has to offer.
