Matt says the main purpose of the disavow tool is to clean up links acquired as a result of doing bad SEO.
Matt Cutts, head of Google's web spam team, post a video about
disavowing links in his latest video where
a user writes in to ask:
Should webmasters use
the disavow tool, even if it is believed that no penalty has been applied? For
example. if we believe ‘Negative SEO’ has been attempted, or spammy sites we
have contacted have not removed links.
Matt says there’s no cause to worry about
disavowing links even without receiving a message about them in your Google
Webmaster console.
In which cases you can use this tool even without a manual action?
1. Once you have ready to drop all other options in
attempting to get those links removed from the web, it would be a perfect time
to use the disavow tool.