Top Cat allows you to specify a main category for your posts. Even though WordPress posts can have more than one category, you may want to specify which of them is the main category for each post. Top Cat lets you do this and also provides template tags for displaying the main category.
Using style sheets, you can then apply different styling to posts based on their main category, even if they belong to the same set of categories. For an example, see my home page at http://www.thunderguy.com/bennett/