The update slightly makes up for the lack of variable refresh rate (VRR) support on PS5 as things stand, though Sony plans to enable that through an update in the future. The fidelity mode prioritizes visuals over performance, offering ray tracing and up to 4K resolution, but the framerate was capped at 30 fps. The performance and performance RT modes have lower resolution, but run the game at 60 fps.
Aiming for 40 fps might seem like an odd number when we're so used to having 60Hz and 120 Hz displays. Because 40 divides neatly into 120, a 120Hz screen will show a new frame on every three refreshes. As notes, that will help ensure Ratchet & Clank Rift Apart looks smooth if you have a compatible display. It'll certainly seem slicker than a framerate that doesn't match up with how often a screen refreshes, which could cause screen tearing.
The framerate issue should become less complicated for PS5 developers once the VRR update arrives. Xbox Series X/S and many PC GPUs currently have VRR support, and more 120Hz+ monitors and TVs are hitting the market.