Arantor

More than that, there are some things that don't have a functional equivalent in any WYSIWYG editor.
The comparison I would point out here is WordPress... that great bastion of using WYSIWYG, until they introduced 'shortcodes' to do things that can't conveniently be done in a WYSIWYG editor.
And if you look, it's functionally identical to bbcode, it's just not called bbcode, but lets you embed things inside posts.
While I wouldn't necessarily argue that they're needed in forum posts, the forum is not the only part of IPB that uses the same editor... where you really might want to support doing things beyond 'what you can do in a WYSIWYG editor'.