The coagulase test is primarily used to differentiate Staphylococcus aureus from other staphylococci because S. aureus produces the enzyme coagulase, which clot plasma by converting fibrinogen to fibrin. This ability is a key virulence factor and helps in rapid identification of S. aureus in clinical specimens. Other staphylococcal species, such as Staphylococcus epidermidis, are coagulase-negative and do not cause plasma clotting. This distinction is critical for guiding appropriate antimicrobial therapy and infection control measures.

Reference: Medical Microbiology, Murray et al., 9th Edition.