For a forum, this only makes sense. Some users will have a fast connection, okay the majority nowadays, and would prefere to set themselves the number of posts to display before a new page.
The admin control panel would be used to set the default posts per page. Then the user could change it to their preference.
In that way, if a forum has a user base of mostly high speed connections, the admin could set the default high, and then the user could choose to downgrade. For those admins where the userbase has slow connections, the admin could set it to a lower number.