Serious question; not trying to be snarky. Other than just personal privacy preferences what would are some practical reasons for why someone would use this phone?
1. I think that I have a right to participate in the digital commons without selling my soul to my choice of two possible corporate masters (Apple or Google).
1A. Corollary, the digital commons necessarily includes the ability to interact with the Internet in a mobile way.
2. There are mobile applications that have such a marked and clear positive impact on my life that doing without them is not merely an issue of convenience, but an issue of capability.
2A. These applications, through market forces, are only available on mobile devices running common mobile OSes.
3. My means of income requires me to fit as much as possible a societal mold that includes availability during reasonable hours via a mobile device.
3A. My employer is not one of the corporate masters, and they have no right to interject themselves in my relationship with my employer that I entered into voluntarily.
When put together, these leave you only one viable option which is running secured hardware with a de-Googled version of Android. I don't see why /this/ device is any better than a Pixel + GrapheneOS done yourself, but the above explains why someone would want a Pixel + GrapheneOS.
You are welcome to disagree with my reasoning, but it is sound.