How Is Luciferase Being Used?

https://pubchem.ncbi.nlm.nih.gov/patent/US-10829770-B2 Why Does the Patent for Fusion proteins containing luciferase and a polypeptide of interest Exist Why Is the new COVID-19 antibody test called SATiN (https://www.verywellhealth.com/bioluminescense-glowing-antibody-testspan-5120623) which uses Luciferase?