In a way, tags in WordPress are like categories. They are meant to group similar topics together in a way that makes them easily accessible (i.e. by clicking on a tag link and seeing all the posts marked with that descriptor).
The problem a lot of people run into is that they end up assigning too many tags to their posts. This defeats the purpose of tags. If you only end up with one or two posts with that tag, it hardly deserves its own “category.”