Displays a count of each commenter's total number of comments (linked to those comments) next to their name on any admin page.
There are many reasons, some of which might include:
Commenters are identified by the email address they provided when making a comment. If commenters are allowed to omit providing an email address, then their name is used to identify them (though this is potentially less accurate).
Since commenters are identified by the email address they provided when making a comment, if they supply an alternative email address for a comment, the plugin treats that email address as a separate person.
Click the "Screen Options" link in the upper-right of the page. It will slide down a form. Click (or unclick) the checkbox for "Comments" to show (or hide) the column.