I am an author on Substack. I moved from Patreon.
Before, I had a mailing list and I had Patreon. A handful of readers supported my work on Patreon thanks to my time on Royal Road.
The Royal Road model (as mentioned above in regards to the author Shirtaloon) is where an author puts up a serial with (typically) daily posts, all free. Readers who get into the serial can then pay a small donation on Patreon for access to the next installments in advance. It's a pay-to-read-ahead model. It doesn't work for everybody, but for those who do make it work, it can be quite lucrative (i.e. Shirtaloon's $30k/mo.).
It requires giving away a lot, for a long period of time. Sometimes a publisher like Aethon (Rhett Bruno's outfit) will pick up some of these more popular serials. At that point the author will remove all but the first 10 percent on Royal Road, continuing with chapters that haven't been published on Amazon yet. When they get to that point they can dip into both pies, KU and Patreon support (again, like Shirtaloon has done).
My sci-fi serial was probably not a good choice for Royal Road because that platform is more fantasy and lit-RPG oriented. Anyway, I learned a lot. But moving over to Substack enabled me to combine pay contributions with my mailing list. So, I can make a newsletter go out to everybody, or I can post the latest chapter in my book to paying members only. It's the same thing I used to do with my mailing list and Patreon, it's just all in one place now.
Finally, I like posting chapters online somewhere because it's yet another place to back things up. And if folks get a sneak peek at what's coming up next, for those interested, that's all for the good too.