Docracy uses document change analysis software to track when terms of service and privacy policies are updated, so any time a site changes its terms, Docracy knows. In most cases, it's just a couple of edits to change the language, but sometimes they are a lot more comprehensive.
Terms of Service change all the time, and while companies usually notify you of the changes, you probably don't actually bother reading through them. This is an easy way to track what has changed, so you can see if it matters to you. Of course, you will need to have at least skimmed the Terms of Service first.