The great thing about the plugin is that you can check any theme that is installed, not just the one that is being used on your live website. This is really useful as it means you can test your current theme against newer versions of WordPress (by setting up a demo area with the latest version of WordPress and checking your current design)
WordPress's standards are so strict that it's doubtful that any theme will pass. I'm currently using a child theme of Genesis. StudioPress always adhere to WordPress guidelines however it still generated a lot of errors.
This plugin was created specifically for theme developers however normal WordPress users may find it useful if their current theme is generating error messages. The same developer also released Plugin Check, a plugin that performs the same checks on WordPress plugins.