I've seen this mod before (on MyBB for certain, not sure if others have it or not) and it has some nice uses that give it an advantage over just using PMs.
Like sometimes you want multiple staff to be able to see a topic and give input on it.
Like for a technical issue, this way it's not a long chain of sending PMs to get help but all support staff check one area. Since some forum software don't have conversation style-PMs like IPB3.x does.
Or on roleplay sites where someone is submitting a character. This way they can submit their character for review without showing everyone (since it may have things they don't want revealed yet but need to be mentioned in order to get approved). This also allows multiple staff to weigh in on a profile to mention things that need fixing/altering. (Since some sites don't let a character get accepted unless two staff review the application. This, according to the theory anyway, prevents some issues.)