High-traffic websites that capture a lot of data from users often encounter performance problems when database input becomes a bottleneck. High volume user-submitted content (comments, ratings, form submissions, etc.) is typically stored in a single (master) database, and this creates problems not only for scale but also for replication and useful backups. It becomes important to be able to write these sorts of things to other secondary storage locations. I'll cover how to successfully write to different databases (MySQL and MongoDB) while still use Drupal's APIs and to cover pitfalls and successes.
In addition to scalability, data sharding provides other capabilities. Applications may be developed using Node.js or with other technology, but still needs access to the same data. With smart data sharding, this becomes possible and even easy.