What exactly do PS5, XBone, Xbox One X users get out of connecting USB mice to their systems?
Better typing experience?
The only time I had a KB+mouse connected to console, is when I had PS3 running YellowDog Linux. Otherwise there's little to no practical reason to do it, even today.
Can they use the mice in gaming?
Only in supported games. Back in my times it was near none, nowadays it's still only few games, which I can count on my fingers.
Unless you need a little competitive advantage in Overwatch - don't bother.
Pretty sure you cannot use mice with any console directly
You
can use it, there's simply not enough games that support it. That questionable overpriced remapper can be a solution, but it's $99 and you still have to program it.
from personal experience I can tell that remapping XY axis on thumbsticks to XY on the mouse, or vice-versa is always a janky process with less than desirable results.
What about 1000Hz+ sampling rates or high DPI mice?
High polling rate not gonna work. PS/XB use bog-standard HID driver, so your mouse will likely be capped at 250pps.
That's also a reason why some high-polling rate keyboards don't work - need to switch it to legacy mode in order to function properly.
DPI switching will work. Swithching DPI on the fly should also work.