I use multiple monitors for audio production. My use case is a bit weird since I code Csound and use a DAW, which is unconventional. It's great for having the DAW up on the 4k, and some code or docs or both on the 1080p, 144Hz. If you didn't guess from the mixture of resolutions and frame rates, I've got gaming covered as well.
Truthfully, the 4k probably has the real estate to do all that on its own, but it was the last monitor I bought and why not use the other? I'm too lazy to figure out a setup to hook up the other 1080s I have lying around. (And don't need the space in any case)