If you want to add faceted navigation to your website, the go to add'on is eea.facetednavigation. There is however a more lightweight alternative if you use collective.collectionfilter. I have used this add'on in combination with a nested keyword add'on we developped at zest, collective.classifiers. Another use case is that we wanted to allow a site visitor to compare a number of graphs/charts visually, without having to integrate the chart data into one 'übergraph' with all data series combined.