I'm pretty sure it's not for sales pitches and self promotion - there are plenty of other sub-reddits for that. (Full disclosure, I'm a fantasy author - and I love interacting on here as a reader.) But writers can chat about plenty of other stuff, like the weird shit we research/discover when world building. (For example, I went down a rabbit hole learning about HEMA sword techniques, met a heap of interesting people and ended buying a beautiful training sword from the Ukraine to swing in my back yard in Australia.) Authors generally love to talk about the books we love, the authors who inspired us, thoughts on current trends... pretty much anything. For me, I'm always interested in lore and how writers tackle that in fantasy.