It's to grate cheese onto the sandwich sandwich.

Crumb catcher.

Around 2018 there were attempts at making gaming mouses lighter and this was one of the ways while still maintaining the structural rigidity. Eventually manufacturers figured out ways to make sturdy, light and still cheap gaming mouses without making so many holes.

The mouse on that picture seems to be the Glorious Model O

More air, less sweaty

Airflow, keeps palms less sweaty.

Texture, provides more tactile feel and better grip, for when palms are sweaty.

Ventilation. Less sweaty hands probably.