Your own site, work, or vendor / client sites could be added.
Or you could want to remove a Comodo (or Honest Achmed's Used Cars and Certificates).
http://www.livehacking.com/2011/04/25/honest-achmeds-used-ca...
https://bugzilla.mozilla.org/show_bug.cgi?id=647959
Just because your OS / browser vendor "trusts" a cert doesn't mean you should.