Drupal is as scalable as the applications it relies on. While there's nothing preventing Drupal itself from being scaled across any number of web servers, scalability of open-source databases such as MySQL and PostgreSQL is an ongoing issue. While these databases do support replication, that's not always enough. For a very small percentage of websites, daily user visits are counted not in hundreds or thousands but in hundreds of thousands.
You have a "master" database that contains data from multiple Drupal sites and you want to share it among them. Normally, you could use table prefixing to allow each of your sites to point to a single table. But what if you do not want content from one site to "bleed" across to the other sites? Let's say you have a network of Drupal sites sharing a user database. You want to share that user's information across your entire network of sites, but only make the information visible from those sites to which the user has subscribed. Or maybe you want to populate baz.com with users who meet some arbitrary criteria. There are lots of possibilities here, but nobody's paying me to write this, so let's get on with it.