It makes it more "cinematic."
Literally the only reason for it to exist I ever heard from game designers.
As far as I am concerned, it does not need to exist; your eyes and brain will create motion blur even without the game itself making it blurry when you move.
I also hate depth of field (again, brain will do this automatically it doesn't need to be simulated), film grain, chromatic aberration (unless I am a robot or literally using a camera), and vignetting (seriously, why does everything have to have a vignette? It only makes sense in a 1st person game where there is something on my face, but they throw this shit onto everything. RDR2 can't even turn it off so thr corners of the screen are basicslly just black the whole time and it looks ugly).