1440 vertical pixels in a 34" 21:9 monitor means you usually don't have to worry about whether your software can adapt to a non-standard DPI. At about 109 DPI, it's the same as a typical 27" 16:9 panel, just with more pixels off to the side. The density is a bit higher than the typical ~94 DPI of a 24" 16:10 display, but the difference can be tolerated by most users. Jumping up to 140 DPI (32" 4k 16:9) is enough that you either need exceptionally good vision, or you need to compensate through software scaling adjustments or major ergonomic changes.
27" iMac has been 2560×1440 from 2009 until they went retina in 2014 (and they're now double in both dimensions, so same logical resolution).
At arm's length, on a 27" screen, 1440p is very practical. Getting more screen space than 1080p is what bumped me from 21-24" screens, and it fills my vision enough that if I jumped to 4k I'd either be moving my head back and forth with a 40" screen, dealing with even smaller UI elements, or wrestling with OS scaling.
Real 4K monitors are effectively the same price and have a lot more pixels. And you can get them in 27", 32" and 34" sizes.
Sure, I understand why the manufacturers want these odd kinds of resolutions so that they can take defective panels and carve them up differently.
However, why would the users want these instead of more resolution? What am I missing?