Different reasons, but I don't really get the point of it either. It only works on Pixel devices, which means you gotta pay iPhone prices and reward Google's bad behavior. If you're fine paying that, you're probably not too far against Google's behavior and thus, why are you using Graphene? If you're against Google's boorish privacy practices, get an iPhone. Apple is sort of trying to take a stand against privacy invasion (at least to Google's scaling; they aren't perfect) but the fact remains, a lot of people don't care about privacy. Pixel+Graphene is objectively better for privacy than iOS, sure, but there are tradeoffs and people who love Graphene are willing to accept them.

The biggest problem with iOS is, it's closed source and we don't know what Apple will do tomorrow. As a Mac guy I don't have a problem with Apple vis a vis my Macs. However the iPhone is kinda silly for a few reasons, but I still prefer it to the alternative because I don't want to be playing around with custom firmware. That's a younger man's game. It was my game when I was a younger man and I don't want to be in that scene anymore. My choice. I know it's a good choice for others. Android isn't really open source either, though. AOSP is — but forks of it, like the Android on Pixels, like OneUI (I still wanna call it TouchWiz), HTC Sense, and all the others, are not. Of course, if you're running Graphene, or Oxygen (again, I still wanna call it Cyanogen), or something like that (I used to be sweet on an AOKP fork called LiquidSmooth), you're playing with open source so you do have that. But you also give up a lot.

I do think it's a bit weird Graphene is only on Pixel. But I guess by keeping the device list small, they can focus on what they want to do, which isn't support every phone, it's supporting ones they know they can.

At the end of the day, Graphene is a better option for privacy than iOS, which is better than any commercial Android OS by default due to not having Google Play Services.

I'd say you have to really assess what kind of privacy you need. Maybe iOS is enough. If it's not, Graphene is a good bet. I don't need Graphene. Heck, I'd be fine with Pixel Android, but I have an iPhone so I can afford to claim the high road in privacy. Just not the highest road. I know where I stand. But I know where I need to be and I'm standing pretty close to it. If someone needs to be in another place, what works for me may not work for them.