The oil seal would seal one face against a rotating member and the other face to a stationary housing. It would prevent dirt from coming between the hub or shaft and the seal.
What are oil seals?
What are Oil Seals, and How Do They Work?
Early engines used O-rings (also called packing rings or toric joints) as seals (first patented in 1896). These are just mechanical gaskets in the shape of a torus (a circular ring--like a lifesaver), seated in a groove and compressed during assembly between two or more parts. It creates a seal at the interface. However, O-rings require a fluid film to lubricate them. They have limited usefulness in vacuum application and at extremes of temperature. The modern oil seal represents a significant improvement over the simple O-ring because it effectively seals in lubrication and prevents contamination from outside under a wide range of pressures and temperatures.
Fitting an oil seal
3. Seal types and numbering system
One common reason leaking occurs in engines is that debris is caught in the fork seal, holding it open and allowing oil to sneak by. So, how do you fix this? This is where the little circular devices come in—oil seals.

