Some of us are huge photography enthusiasts, and while the non-pros have come closer, they still can't touch the Pros. iPhone 11 bokeh is a joke compared to the 11 Pro bokeh.
Premium materials and 4x4 MIMO is also exclusive to the Pros usually. It's nice to hold a premium phone instead of a plasticy one, and for some people $300 is not that much money considering the tradeoff of using that phone for the next year or two or three (or more?) without a premium feel.
Mark my words I'm fairly certain the 12 non-Pro will sport the same plasticy look of the 11, and will be just as slippery to hold caseless unlike the new Pros which will be the only phones to get the squared new look for now.
Pro models also have a screen with even deeper blacks, as well as 6GB ram instead of just 4GB like the non-pro 12s will likely have.
I also wonder if this year the 12s will still come with only 64gb of space compared to the Pros starting at 128gb at the lowest now.
All of these little things start to add up...
It's a bit like going for the SS or Edition Apple Watches instead of the Aluminum ones, though not completely since in that case you are solely paying the extra money for the appearance and durability, no functionality even.
It's not necessary, but it sure is nice to have premium materials, and to not have a screen that is scratched so easily in the case of the Watch.
To me the money is worth spending on these devices I use nearly all of the time, but ymmv.