When you cluster things that are similar before analyzing them. For instance, you go to a frozen yogurt place. You order without clustering. You get a disgusting, multi-colored mush of flavors. If, instead, you use some cluster analysis before ordering, you might see that you could mix chocolate and peanut butter flavors, or mix the raspberry and triple-berry flavors for a fruity explosion...now, you have two balanced clusters, which satisfy both your need for chocolate-y-ness, and fruitiness. Nice!
Investors use cluster analysis to build balanced portfolios. For instance, stocks that behave similarly can be grouped together, and then you can balance those groups for a well-rounded portfolio. Cluster analysis helps investors build portfolios that are diverse and not redundant.