Mechanical seals prevent leaks of the pumped fluid and the loss of expensive pumped products.
They keep or seal the pump so that the fluid stays inside it. Since about the 1950s, mechanical seals have almost completely eliminated the inefficient and expensive stuffing box.
They are installed where the pump shaft enters or leaves the pump housing. Seals of various types, sizes, and configurations are available.
However, they all use the basic principle of a combination of fixed and rotating surfaces.