The black goo is bitumen, or tar. It is the binder used in road asphalt and was a common pitch used to seal boat hulls. The article focuses on the ritualistic aspect but doesn’t focus on the functional reasons. Sealing the mummified remains from the elements, sealing in the gasses and smells from mummification/decomposition, and gluing the sarcophagus seem like reasonable functional explanations.

Some statues are also covered with pitch so maybe it was also used as a theft deterrent. The pitch covered statues are the best evidence that the pitch was ritualistic rather than functional, in my opinion.