By listing all the readers, means who view the thread, this helps the administrator and also the content producers sense customer tastes serving the purposes of sales increase.
After you install the add-on, go to User Group Permissions, choose the group that you would like to show the thread readers list and under Forums you will see 2 new settings called:
1) Can view who read this thread for all threads:
2) Can view who read this thread for own threads only:
You can also set the permissions on a node per node basis.
Then when viewing a thread, just below the thread title you will see a link that said: Thread readers. Click it and a new pop up page will open containing all users that read that thread. The thread readers will be counted from the moment that the mod is installed.