It's mainly for tagging, taxonomies and UX journeys. You can use the category on Pages (and add them to blocks) and them use this as complex meta data against the IContent. E.g you might create a set of categories for pages that group them in to these taxonomies and then display them on a search page as filters, using these categories to then filter your search content down. Using categories allows these to all be flexibile and CMS managed. 

One thing we do is create an Article page type and a article type parent category with event, blog, news and then users just create an article and tag the content type. We then create a generic set of article listing blocks that have the category property exposed and if the user selects them on that block we filter the content using Episerver Find to only show those types.

It's a very powerful feature, you can effectivly slice any of your content in to these taxonomies then create flexible listing that can be set up for users to be able to select what they want. In the UX phase of a website build usually the IA structure of a site is all taxonomied, so if you use this you can make very flexible blocks/components. 

Also the new Advnce personalisation system can hook in to this to create personalised user journeys for customer.