(Not op)
it typically offers better video processing and upscaling, more accurate color reproduction, cleaner gradients, and superior HDR handling (including dynamic tone mapping on some models). Many also support Dolby Vision from UHD Blu rays, which the PS5 does not.
It won't show on a bad screen that much, but a dedicated player will squeeze out more of the disc.
Knowing nothing about the topic: what kind of processing and upscaling happens when I play a 4k movie on a 4k TV?