Meta keywords are like "tags" but only for search engines - they're embedded in the HTML document, but visitors don't see them visually.
Tags can also be useful for SEO purposes, but are primarily a means to allow end users to cross-link similar content. If your community is about pets, people might use tags like "cat", "dog", "hamster", etc. When reading a topic about a hamster, I could click the tag and see all other topics that also have a "hamster" tag to see content I may be interested in.
You can simply disable tags in the AdminCP if you don't wish to use them. This is a better option IMO than allowing tags to be used but not displaying them to the end user.