What purpose used in mechanical seal?

to prevent leakage of the oil or water or even the air from the enclosed mechanical circuit to the other components, or to isolate the mechanical circuit off the other parts of the mechanical system

and example from it is the oil seal which prevent the oil leakage to the atmosphere or the other mechanical components and keep the circuit oiled to decrease the friction of the mechanical components

It's a device used to join systems together by preventing leakage