It’s almost impossible to get accepted by any of the big publishing companies. Only 1 in 1000 authors who try succeed. Some say, it’s rather 1 in 10000.
So in almost all cases the benefit of writing and (self) publishing isn’t making a living of it or getting famous. It’s about improving your writing skills. It’s about improving self discipline and focus. It’s about thoroughly researching multiple subjects related to the book. It’s also about experiencing flow i.e. the “mental state of operation in which a person performing an activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. In essence, flow is characterized by complete absorption in what one does.” This means it feels good. Often it’s like during and after intense exercise.
I just sold about 300 copies of my book and I estimate that less than 1000 people have read my novel (used copies get resold on Amazon frequently). There are just too many novels out there. It’s easier to sell non fiction, but still hard, especially for non-niche subjects.
One of the main benefits for me was this: A lot of influential people in my company know I pulled this off. English is my second language and it’s extremely important when you work in large companies in Germany. It’s a huge plus to be seen as someone who is close to native speakers. The other thing is about establishing a reputation for perseverance. Don’t underestimate this. A lot of people run marathons just for the sake of it. When they succeed they feel good about themselves. And others will learn about this. Writing is a bit like running a marathon. Including all the wonderful endorphins.