Phillip Caudell wanted a second high-resolution display for his iMac. Unfortunately, Apple's own model is wildly expensive and he didn't like the look of the plastic alternatives. So he did "what any normal person would do … and built my own."
All told he's out £530 (~$740), easily enough to grab a used LG UltraFine 5k, so it's certainly an aesthetic rather than an economical project.
As an aside, isn't it a shame Apple no longer lets iMacs be used as external displays? That was always a great feature of the "polycarbonate" era.
I often get the bug to turn an old Apple Cinema Display into a fully-fledged all-in-one, so pleasing do I find its design. And you could fit a *lot* of modern computer inside that ~4cm thick case. But high-definition panels just aren't made at reasonable prices at the right sizes to make for a perfect fit.