Actually, it is a seal for the splash shield, not the WB seal Factory-type WB seals have a deflector built in). For performance use, toss both the shield and the seal, the majority of cars in the last 20 years don't have 'em (but on some SUVs, 4X4s, etc.). For lots of dirt-road, sloppy-weather driving, the shield can help reduce pad wear by keeping grit and dirt away.

Rick