At the time I switched, the built-in blocker worked on a site I regularly used while Firefox+ublock did not (I think it would just prevent things from working or cause infinite ad-loops). If I wasn’t looking for an alternative adblocker, would probably have never bothered switching. There’s also the “get pocket change from using our browser” thing. Some may have been speculating on the value of BATs?
The only reason I use it over firefox is about tab grouping and how tab mutting work by default. I don’t feel like trying out a bunch of extensions to find one that does what I already get from another browser. Also don’t have to worry about installing ad blocker. Originally switched because it worked better than uorigin for a specific use-case that was relevant for me. I also have vivaldi, firefox, and librewolf install and will use them occasionally. Privacy isn’t a big concern for me though; when I tried to switch to librewolf, the privacy features ended up annoying me so I disabled a lot of them because they interfered with using the browser how I wanted.
Not recommending Brave. I agree at least in theory with using Firefox and I want more people to use Firefox. But its what I’m use to and there was reason for me to try it out at the time I switched to it (that’s probably irrelevant now).