There might be various reasons behind this, but the most obvious is that they need help launching and operating Google Games, an online hub which Google is intendindg to launch soon. The profile of the site will be providing online social games, very much like Zynga does embedded in Facebook. The existance of Google Games have not yet been acknowledged by Google, however, TechCruch says they confirmed it from multiple sources.
The other main reason behind the decision is if the US Legislation allows online poker in the United States (which is highly debated whether they will), Google will have a very good position and a decent market if they couple up with Zynga. Many online gambling industry experts have already speculated that Google and Facebook will be among the biggest threats to current online poker casinos and sports betting firms, while brick and mortar businesses like Harrah’s are much less of a worry for them.
Google seems to be up to something big, but we yet have to see what will turn out of their cooperation with Zynga.
