I haven't used gitlab's wiki specifically, but the reason why you would use something like confluence is because you get a wiki with a wysiwyg editor, so less technical people can use it without having to learn markdown and there is less friction to making docs. It's not specific to confluence.

Hell google docs would probably beat confluence in a corp if they let you make a wiki structure out of it vs. it's current 'pile of documents' organization model because it has a better wysiwyg editor and inline comment system.