Following a person who hasn't posted a tweet isn't very interesting. Likewise, user's without a profile picture could be spammy or robot created accounts. You can search for either of these scenarios singularly or mix both for a real hardcore spam search.
Find and locate people who aren't following any Twitter users at all. Such accounts aren't going to read your tweets or anyone else's, for that matter.
Quickly congregate a list of people you follow who have the magic blue tick and are using verified Twitter accounts.
You could be following people who haven't posted a tweet in ages. Using the search by date option, you can quickly locate user's whose last tweet was after a selected time. Select from days, weeks, months or years.
Here's a quick overview of the tool and its features