I’ve seen many people that ignore tags on blogs. Tags are used to describe what the post or page is about. WordPress creates a page for every tag you use, when an user goes to a tag page he will see all articles that contain that tag. It is something like keywords.
Even if you use very descriptive categories for your blog, you can still take advantage of using tags. For example a visitor reads one of your posts, he likes it and he want to read more about that subject, at the end of you post you have related articles shown, but if you follow a link from related articles, you read that post but the related articles in the next post are the same as the first article. So can read max 6 articles on a subject using related links navigation. Maybe someone want to read everything about a subject, but he find out that in a category listing page he got more messages than he wants and he will like to have a filter. Adding tags to your articles you can tell to your visitors how an article is related to the other, which tags they have in common. At the end of an article he can decide what topic he want to read more.
If you want to save time but having tags too, you can find few automated tags plugins.
The good aspect of tags is that you can display them in different ways. You show tags under an article, you can have a menu list with most used tags on your blog, but the best way that i recommend is to use a tag cloud.
The advantage of a tag cloud is that your visitors can see what your articles are focused on, or what subjects do you approach. It can also work as a filter, go to a tag link so he can read only articles about that thread.