Anthologies make a lot of sense for writers. An anthology is a large book filled with short stories and novellas all surrounded a single subject. The stories are all generally done by different authors, so the readers get a variety of writing styles for one low price.
Financially, it’s great for the author. He puts together a small story that gets put into a print or e-books and he splits the profits with the rest of the authors. It’s not going to make him rich, but it he also didn’t spend a year or 6 months working on it. He sat down one weekend, banged it out and there you go.
Science fiction anthologies can be about any subject from alien species, tributes to great authors like Ray Bradbury or wormholes. The stories are all different, but they have something to do with the overall subject of the anthology.
Anthologies are popular with fans because you are opening up the fan base. Unlike a book, which has the fan base of a single author, you’re opening it up to a dozen or more. The fan gets stories from his favorite science fiction writer and may end up enjoying the style of some of the other writers as well.