You can promote your work at exhibits.
Award winners will have their work promoted by their peers.
Community and Industry Insight
There are magazines or newsletters at varying times throughout the year, each having a style and specific focus. Most discuss craft, industry news, and promote members. Local chapters offer an opportunity to connect with fellow writers in person, but people can also connect through social networks and online forums.
Every writer is a human, and every human needs a support group or social group. Sometimes, this support group or social group is one's own family. Sometimes, this group is one's own extended family. Sometimes, this group is mostly friends. For people who do not want to share with family and friends, they have to seek a support group beyond the home. So, they invest in writing organizations as a way to promote writing, increase sales, improve craft, and socialize with other writers. They want to be with like-minded people who care about their work, and if they can't get that from family and friends, then they will get what they want from society. Someone will care.
Personally, I think fictional writers work in the Entertainment industry. They write novels to entertain the masses. They may inspire or start critical discussions, but they need to entertain as well. If the reader finds the work boring/unentertaining, then the reader will close the book and quit reading. As such, they function like entertainers. Their livelihood depends on their ability to entertain. After publishing, the book needs to be promoted. People need to know about it to make a sale, and if Book 1 sells well, then that increases the author's reputation to make more books. Because it does take a lot of time and energy to write a book and publish that book and promote that book, writers are generally very secure with their lives. They are usually literate, because you need to be literate in order to write. They are usually privileged, because of their own literacy. The society in which they live is relatively stable enough to promote the arts. War and political instability are really bad times to write a novel. Poverty means you have to get a job to feed your family rather than sit down and think about a story. Hopefully, that can give you an idea what kind of people join these writer's organizations, and if such an organization is right for you.