Tagging a post makes it easier for a reader that is searching for something in particular. They can select a tag, and then display all posts with that tag. That’s it. I’d imagine that it also influences search terms from sites like Google, but cannot say for certain.
Anyway, tagging, like so many other things we’ve looked at this semester, serves a valid point, but must be used properly. If you take the few seconds to tag your post, you’ll make it much easier for readers.