More or less? 'Because I want people to read it, no matter which side of the divide they're on.' Just because one is on both isn't going to be the case with others. And people are, by and large, as lazy as they can make it. So those who are keeping to one side or the other of the divide, they won't see X. So to those who care, crossposting is their 'best of both worlds'.

Simply enough, it doesn't matter what it does in the macro, so long as it covers their situation in the micro.