
The Recipe module is probably a good example. On one site, in order to foster a bit more "community" feel and encourage visits, the administrator decided to add a group cookbook (a real one, not like this book). The recipe module does that.
It didn't take very long to realize that entering a bunch of recipes without any organization would get messy pretty quick. Well, recipes fall into several categories: Appetizers, Entrées, Desserts, etc. So let's set up those things as a "vocabulary" with which we can organize the recipes.