In general, the administrator of a site usually wants to open his/her site as much viewers as he/she wants. However, with some special threads, they will exhibit new users and just want the official members to see it. So they might could use this add-on "Limited Guests on Thread Viewing".
By setting a limitation on the number of guests who are viewing a thread, they will encourage this kind of audiences going to register to be a member of site. This action will stimulate others and raise the number of members into a high level.
You can enable and disable the mod for each group separately, set the maximum number of views separately for guests and and users waiting email account confirmation, enter your own separate custom error message for both groups after they reach the maxmium views. By default it is set to 5. You can also exlude forums from the view limit.
The error message will also show when you mouse over the threads after the maximum number of thread views has been reached.