The redundant GND pins are necessary for signal integrity and low EMI when running at higher speeds; the high speed signals need a return current path with as little loop area as possible, so you want those signal pins close to GND on the connector.
On modern CPUs there are tons of "redundant" ground pins sprinkled all over[2]. Just about every high-speed GPIO pin and power pin has it's own ground pin right next to it.
Keep in mind "high speed" mostly relates to having fast edges. You can have EMI issues with a "slow" 1MHz signal if the edges have rise/fall times are say a few ns, which modern microcontrollers can do.
[1]: https://datasheets.raspberrypi.com/rp2040/rp2040-datasheet.p... (1.4.1)
[2]: https://en.wikichip.org/wiki/amd/packages/socket_am5#Pin_Map