Collections are often used for this and the book brings attention to that charity and the writers supplying the stories. In the words of Del Boy "everyone's a winner". It's a great way of supporting a cause and trying out different authors' works. And I just love story collections getting publicized as while I have no problems with e-books and the like, I would not like to see the death of the paperback so to prove there is very much still a market for the latter is a good thing.
They're a great way of authors to develop a writing CV...
And you kind of get a small community of writers involved in the book in question, which encourages co-operation to publicize it and hopefully develops friendships which go on long after the life span of the book. Being published like this is a great confidence builder, a reminder "you can do it" and gives you something to put in your submission letters to agents/publishers. Book collections are something you can be proud about taking part in. Well that's been my experience anyway.
They cover all the emotions...
I prefer the humorous story collections but horror books have their own devotees and collections can be a great way of testing a whole genre out, which must be a good thing. I hope (and believe) other book sales happen later due to the launch of collections and initial interest in them. And anything that also encourages the form of the short story to not just survive but thrive also has to be good for literature as a whole.