I think GPs point is if you need more horsepower you might as well get a used small form factor, and if you care about the GPIO and other interfaces you might as well get a PI 3 or 4. I get what you're saying it's a computer it could do all sorts of stuff.
I think the stronger argument/usecase is it's a drop-in replacement for the pi4. So if you're already dedicated to that form factor...
For anyone building a little homelab or whatever, especially in the US where triple the (low) power consumption from 3 to 9W+ isn't a big deal, a used SFF/mini PC is a better value proposition. Especially if you want to add storage, 2.5 Gbps networking, etc.
But for people integrating a computer into a larger project (robotics, automation, controls, etc.), or buying a little 'IoT' device to tinker with, buying used gear that is often much larger and usually requires a large external power brick might be a turn-off.
I think the stronger argument/usecase is it's a drop-in replacement for the pi4. So if you're already dedicated to that form factor...
pi5 number go up