Categorizing and tagging your videos and posts make it easier for you to organize and gather all your asset with the same category.
You may want to apply changes to videos or posts within a certain category value, or simply view all content tagged with a certain category value. Because category values act as tags, you can filter by category value.
But, categorizing is mainly used to create your playlists. You can configure playlists to automatically bring your video content based on categories you've created.