You're confusing the two. "Display in positions" is the pages you want the widget to show on. The following would show on the main forum list, when viewing a forum, and when viewing a thread.
View attachment 278962
The display condition in this case targets guests. You need to place a widget where a position is not already defined, that's when you'd manually add the
<xf:widget key="widget_key" />
You could also use "Public sidebar" but that goes on every single page which sometimes can cause problems (members page for example has a left column so it turns it into a 3 column layout).