Social gaming requires a large amount of data to be stored on the server side as most of the future game design decisions depend of the user data crunching. Zynga is one of the biggest social gaming companies with over millions of users for each game. Storing that amount of data is a big deal, and persisting it at the same time can be a large overhead.
Zynga uses a concept of “eventual persistence” in which a layer of cache is added, in this case memcache, where all the data gets stored. This layer is more robust than usual cache layers. This data can be persisted for long times in this cache layer and then is transferred to the database servers in one pass. More details of the structure of backend of Zynga engineering can be found here.