When you activate the "Allow users to sort forums" switch, this places an area on your index page above the forum list where the visitor can select which field to order the list by, and a similar area over subforums on the display topics in forum page, and on the search forums results page.

When you activate the "Allow users to sort topics" switch, this places an area on your 'display topics in forum' page above the topics list where the visitor can select which field to order the list by, and also on the search topics results page.

The visitor doesn't have to make a selection, it defaults to the order you've set at Admin -> Settings -> General. Still, you may prefer to switch off the option in order to simplify your site's visual style or to force a uniform result order for all.