Some HTML errors are outside of the control of web developers. For example some frameworks and content management systems generate invalid HTML by adding obsolete elements or attributes. This validator lets you filter out these errors so that you can focus on the errors that you do have control over. Using filters effectively will ensure that the errors you need to know about don't get lost in the noise of framework/CMS errors.
To learn more, read my blog post Filtering HTML5 Validator Errors.