I've got the point. Thanks for all the help.
What this setting does?
I'm having problems with infinity scroll, and I'm looking for good options to disable it for forum.
If you (from user-perspective) have problems with infinite scrolling, you may only disable it within your users settings rather than within ACP.
But if you think the common user does like pagination more than infinite scrolling (so do I), you can do this with the option you took a picture of.
If you (from admin-perspective) have problems with infinite scrolling you may have to explain it in more detail since as of I know there is no option to completely disable infinite scrolling feature.
This ACP setting just defines the default value for new users.
I think, ACP should provide option to use only one method for your users, so they will not have an option to select type of pagination, if such decision was made by administrators.
I will be happy to use infinite scroll, when this feature will work really well.
@Nicolas why would you want to remove that option from end users?
@pitaj I think for the same reason you can disable themes for users, To have a particular style for a website and not letting the user mess with it in case of bugs.