Imagine a situation where one day you have to set up a distributed cluster with the use of Solr. The amount of data is just too much for a single server to handle. Of course, you can just set up a second server or go for another master server with another set of data. But before Solr 4.0, you would have to take care of the data distribution yourself. In addition to this, you would also have to take care of setting up replication, data duplication, and so on. With SolrCloud, you don't have to do this—you can just set up a new cluster, and this recipe will show you how to do that.
Before continuing further, I advise you to read the Installing ZooKeeper for SolrCloud recipe from Chapter 1, Apache Solr Configuration. It shows you how to set up a Zookeeper cluster in order to be ready for production use. However, if you already have Zookeeper running, you can skip this recipe.