What do you use cluster analysis for?

Cluster analysis is useful for analyzing extremely large data sets by splitting them into more manageable chunks $-$ or clusters $-$ to be processed in parallel by multiple computers. When data is broken up and processed that way, each cluster can be processed much faster.

Cluster analysis is useful for classifying entities, such as people, into clusters $-$ or segments $-$ that are as homogenous as possible in terms of relevant traits. It does that by putting each entity from a data set in a single group along with other entities that are as similar as possible.