They cover the cutouts in your rim and — if you’re running a tube — they protect it from spoke heads and other sharp matter (okay, so maybe they have TWO jobs but they do them both really well.). PVC is great if you plan on only running tubes.
WIQUYBML 700c Road Bikes Wheelset Carbon Bicycles Rim 24 Hole 29" C/V Brake Disc Wheels Thru Axle Hub for 7/8/9/10/11/12 Speed Cassette 1700g
WIQUYBML 26/27.5/29 Inch Mountain Bike Front Fork, Straight Tube Line Control, Tortoise-Rabbit Dual Air Chamber Shock Absorber Suspension Fork A,26inch
WIQUYBML 26/27.5/29 Inch Mountain Bike Fork, Rebound Adjustment, Tortoise-Rabbit Dual Air Chamber Shock Absorber Front Straight Tube Line Control, Suspension Fork B,29inch
WIQUYBML 26/27.5/29 Inch Mountain Bike Fork, Rebound Adjustment, Tortoise-Rabbit Dual Air Chamber Shock Absorber Front Straight Tube Line Control, Suspension Fork A,27.5inch
HYJAUSXB 26/27.5/29 Inch Mountain Bike Fork, Rebound Adjustment, Tortoise-Rabbit Dual Air Chamber Shock Absorber Front Straight Tube Line Control, Suspension Fork A,27.5inch
HYJAUSXB 26/27.5/29 Inch Mountain Bike Front Fork, Straight Tube Line Control, Tortoise-Rabbit Dual Air Chamber Shock Absorber Suspension Fork B,27.5inch