Is it just payment? Well, no. I've known established writers to submit to non-paying markets simply because they liked the look of them. I myself have occasionally seen a market and, becoming obsessed with it, written a story specifically, despite the pay. Of course, professional paying markets matter if you live off your short fiction or you want to apply to the SFWA/HWA, but not if you write simply for the love of it.
Is it exposure? This, too, is a difficult one, because a short story magazine with a distribution of 1000 or so doesn't provide much exposure to someone like Clive Barker. Furthermore, why would he need further exposure anyway? Surely his readers know of him and new readers who're looking for genre work will know to consider him.
Perhaps it's being 'part of the club'. If you're published at a given venue, you become part of a cabal of writers who have all been published there. Appearing alongside writers you admire, or in a beautifully designed and highly selective magazine is always a good thrill, whether or not you need the exposure. So perhaps all writing is selfish and all publishing is vanity, but this probably comes as no news to most writers, who've secretly been hiding this info from the rest of the world and making them dependend on us for, well, everything :D