Mathew,
The easiest way to become familiar with categories is by mounting a blogger module.
Example
1. Once mounted go to Blogger admin.
2. Select "site wide categories" and start adding some that refer to the kind of topics you want to feature.
3. In 5.15 categories are used by blogger, logger, project manager. I assume in next releases more packages will benefit from categories.
Talking about reuse of functionality, form a user point of view it's one of the most powerfull services and a great timesaver.
I am currently using categories to create a functionality weblog for end users, where no longer modules are king, but all available features are explained.
Cheers
