Rails plugins vs discourse plugins - what is the difference?

I read the beginners plugin guide and this . Is there something else? I also looked at the code of some plugins. I just know that I need tables. And there are other plugins as well that use their own tables. Why is there this obsession with not creating new tables? Is there a big cost involved with creating new tables? Especially if they are namespaced with the plugin name at the beginning I really dont see the downside.