WordPress is a very powerful content management tool with lots of ways to slice and dice your content. When you have a large number of posts it can become difficult to sort them. Well, categories to the rescue.
When you categorize your posts you are essentially putting them into buckets. And you can apply more than one category to any post. When you have them categorized properly then you can easily filter by category to make them more manageable.
Here’s a short video about how to filter your posts by category.